Manufacturing method of semiconductor device comprising oxide semiconductor

ABSTRACT

A semiconductor device using an oxide semiconductor is provided with stable electric characteristics to improve the reliability. In a manufacturing process of a transistor including an oxide semiconductor film, an oxide semiconductor film containing a crystal having a c-axis which is substantially perpendicular to a top surface thereof (also called a first crystalline oxide semiconductor film) is formed; oxygen is added to the oxide semiconductor film to amorphize at least part of the oxide semiconductor film, so that an amorphous oxide semiconductor film containing an excess of oxygen is formed; an aluminum oxide film is formed over the amorphous oxide semiconductor film; and heat treatment is performed thereon to crystallize at least part of the amorphous oxide semiconductor film, so that an oxide semiconductor film containing a crystal having a c-axis which is substantially perpendicular to a top surface thereof (also called a second crystalline oxide semiconductor film) is formed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method formanufacturing the semiconductor device.

In this specification, a semiconductor device means any device which canfunction utilizing semiconductor characteristics, and includes in itscategory an electrooptic device, a semiconductor circuit, and anelectronic device.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film formed over a substrate having an insulatingsurface (the transistor is also referred to as a thin film transistor(TFT)). The transistor has been applied to a wide range of electronicdevices such as an integrated circuit (IC) and an image display device(display device). Although a silicon-based semiconductor material iswidely known as a material for a semiconductor thin film applicable to atransistor, an oxide semiconductor has been attracting attention as analternative material.

For example, a transistor whose active layer uses an amorphous oxidecontaining indium (In), gallium (Ga), and zinc (Zn) with an electroncarrier concentration of less than 10¹⁸/cm³ is disclosed (see PatentDocument 1).

REFERENCE

-   Patent Document 1: Japanese Published Patent Application No.    2006-165528

SUMMARY OF THE INVENTION

However, the electric conductivity of an oxide semiconductor is changedby deviation from the stoichiometric composition or entrance of hydrogenor moisture, which forms an electron donor, in the oxide semiconductor.Such a phenomenon leads to a change in the electric characteristics of atransistor using the oxide semiconductor.

In view of the foregoing, one object of one embodiment of the presentinvention is to provide a semiconductor device using an oxidesemiconductor with stable electric characteristics to improve thereliability.

In a process for manufacturing a transistor including an oxidesemiconductor film, an oxide semiconductor film containing a crystalhaving a c-axis which is substantially perpendicular to a top surface ofthe oxide semiconductor film (also referred to as a first crystallineoxide semiconductor film) is formed, and oxygen is added to the oxidesemiconductor film to amorphize at least part of the oxide semiconductorfilm, so that an amorphous oxide semiconductor film containing an excessof oxygen is formed. Then, an aluminum oxide film is formed over theamorphous oxide semiconductor film, and heat treatment is performedthereon to crystallize at least part of the amorphous oxidesemiconductor film, so that an oxide semiconductor film containing acrystal having a c-axis which is substantially perpendicular to a topsurface of the oxide semiconductor film (also referred to as a secondcrystalline oxide semiconductor film) is formed.

As the method for adding oxygen (containing at least one of an oxygenradical, an oxygen atom, and an oxygen ion) into the first crystallineoxide semiconductor film, an ion implantation method, an ion dopingmethod, a plasma immersion ion implantation method, plasma treatment, orthe like can be used.

The oxide semiconductor film containing a crystal having a c-axissubstantially perpendicular to a top surface of the oxide semiconductorfilm (hereinafter also referred to as a crystalline oxide semiconductorfilm) has neither a single crystal structure nor an amorphous structure,and is a crystalline oxide semiconductor having a c-axis alignment (alsoreferred to as a c-axis aligned crystalline oxide semiconductor(CAAC-OS)). The crystalline oxide semiconductor film enables a change inthe electric characteristics of the transistor due to irradiation withvisible light or ultraviolet light to be further suppressed, so that ahighly reliable semiconductor device can be provided.

According to one embodiment of the present invention disclosed herein,oxygen is added into a first crystalline oxide semiconductor (CAAC-OS)film to amorphize at least part of the first crystalline oxidesemiconductor film to lower the crystallinity, and then, heat treatmentis performed thereon to recrystallize the oxide semiconductor film, sothat a second crystalline oxide semiconductor (CAAC-OS) film is formed.The first crystalline oxide semiconductor (CAAC-OS) film and the secondcrystalline oxide semiconductor (CAAC-OS) film are oxide semiconductor(CAAC-OS) films each containing a crystal having a c-axis which issubstantially perpendicular to a top surface of the oxide semiconductorfilm. Since the first crystalline oxide semiconductor (CAAC-OS) film isthe crystalline oxide semiconductor (CAAC-OS) film containing a crystalhaving a c-axis which is substantially perpendicular to the top surfaceof the oxide semiconductor film like the second crystalline oxidesemiconductor (CAAC-OS) film, the crystallinity of the secondcrystalline oxide semiconductor (CAAC-OS) film obtained through oxygenaddition and recrystallization can be improved.

The oxide semiconductor film (the amorphous oxide semiconductor film,the second crystalline oxide semiconductor film) has a region containingan excess of oxygen as compared to the stoichiometric composition ratioof the oxide semiconductor in a crystalline state, owing to the oxygenaddition. In that case, the oxygen content is greater than that in thestoichiometric oxide semiconductor or higher than that in the singlecrystal state. In some cases, oxygen exists between lattices of theoxide semiconductor. The composition of such an oxide semiconductor canbe expressed by InGaZn_(m)O_(m+3x) (x>1). For example, supposing thatm=1, the value of 1+3x in InGaZnO_(1+3x) (x>1) exceeds 4 in the casewhere the content of oxygen is excess.

In an oxide semiconductor film, oxygen may be eliminated to form anoxygen vacancy. An oxide semiconductor with no excess oxygen cannotrepair such an oxygen vacancy with other oxygen. In contrast, since thesecond crystalline oxide semiconductor film according to one embodimentof the present invention is a CAAC-OS film containing an excess ofoxygen, the excess of oxygen (which is preferably excess as compared tothe stoichiometric composition ratio) contained in the film can act torepair an oxygen vacancy in the second crystalline oxide semiconductorfilm immediately.

The aluminum oxide film provided over the oxide semiconductor film has ahigh shielding effect (blocking effect) of blocking penetration of bothoxygen and impurities such as hydrogen, moisture, a hydroxyl group, andhydride (also referred to as a hydrogen compound).

Therefore, in and after the manufacturing process, the aluminum oxidefilm functions as a protective film for preventing entry of an impuritysuch as hydrogen or moisture, which causes a change in characteristics,into the oxide semiconductor film and release of oxygen, which is a maincomponent material of the oxide semiconductor, from the oxidesemiconductor film.

Further, since the heat treatment for crystallizing the amorphous oxidesemiconductor film is performed in the state where the amorphous oxidesemiconductor film is covered with the aluminum oxide film, oxygen canbe prevented from being released from the amorphous oxide semiconductorfilm by the heat treatment. Thus, the resulting second crystalline oxidesemiconductor film can maintain the amount of oxygen contained in theamorphous oxide semiconductor film, and therefore has a region where theamount of oxygen content is excess as compared to the stoichiometricoxide semiconductor in a crystalline state.

Therefore, the second crystalline oxide semiconductor film has highpurity because impurities such as hydrogen and moisture do not enter thesecond crystalline oxide semiconductor film, and has the region wherethe amount of oxygen is excess as compared to the stoichiometric oxidesemiconductor in a crystalline state because oxygen is prevented frombeing released therefrom. Accordingly, the crystalline oxidesemiconductor film enables a variation in the threshold voltage V_(th)of the transistor and a shift of the threshold voltage (ΔV_(th)) due toan oxygen vacancy to be reduced.

Before the aluminum oxide film is formed, it is preferable to performheat treatment for dehydration or dehydrogenation on the firstcrystalline oxide semiconductor film so as to remove a hydrogen atom, animpurity containing a hydrogen atom such as water, and the like from theoxide semiconductor film.

By removing hydrogen from the oxide semiconductor to highly purify theoxide semiconductor so as not to contain impurities as much as possible,and repairing oxygen vacancies therein, the oxide semiconductor can beturned into an i-type (intrinsic) oxide semiconductor or a substantiallyi-type (intrinsic) oxide semiconductor. That is, with the removal ofimpurities such as hydrogen and water as much as possible and repair ofoxygen vacancies, a highly purified i-type (intrinsic) or substantiallyi-type (intrinsic) oxide semiconductor can be realized. Thus, the Fermilevel (Ef) of the oxide semiconductor can be changed to the same orsubstantially the same level as the intrinsic Fermi level (Ei).

One embodiment of a structure of the present invention is a method formanufacturing a semiconductor device in which a first crystalline oxidesemiconductor film containing a crystal having a c-axis which issubstantially perpendicular to a top surface of the film is formed overan insulating film; a gate insulating film is formed over the firstcrystalline oxide semiconductor film; oxygen is added to the firstcrystalline oxide semiconductor film through the gate insulating film,so that an oxide semiconductor film at least part of which is amorphousis formed; a gate electrode layer is formed over the gate insulatingfilm; an aluminum oxide film is formed over the gate electrode layer;and heat treatment is performed thereon to crystallize at least part ofthe oxide semiconductor film at least part of which is amorphous, sothat a second crystalline oxide semiconductor film containing a crystalhaving a c-axis which is substantially perpendicular to a top surface ofthe second crystalline oxide semiconductor film is formed.

One embodiment of a structure of the present invention is a method formanufacturing a semiconductor device in which a first crystalline oxidesemiconductor film containing a crystal having a c-axis which issubstantially perpendicular to a top surface of the film is formed overan insulating film; a gate insulating film is formed over the firstcrystalline oxide semiconductor film; a gate electrode layer is formedover the gate insulating film; oxygen is added to the first crystallineoxide semiconductor film through the gate insulating film, so that anoxide semiconductor film at least part of which is amorphous is formed;an aluminum oxide film is formed over the gate electrode layer; and heattreatment is performed thereon to crystallize at least part of the oxidesemiconductor film at least part of which is amorphous, so that a secondcrystalline oxide semiconductor film containing a crystal having ac-axis which is substantially perpendicular to a top surface of thesecond crystalline oxide semiconductor film is formed.

In the above-described structure, the first crystalline oxidesemiconductor film containing a crystal having a c-axis which issubstantially perpendicular to a top surface of the film can be formedby any of the following methods: an amorphous oxide semiconductor filmis formed over the insulating film, and is at least partly crystallizedby performing heat treatment thereon; film formation is performed overthe insulating film while being heated.

Further, a top surface of the insulating film in a region which is incontact with the oxide semiconductor film preferably has less roughness.Specifically, the average surface roughness of the top surface of theinsulating film is preferably greater than or equal to 0.05 nm and lessthan 0.5 nm (or greater than or equal to 0.1 nm and less than 0.5 nm).Such a top surface of the insulating film with less surface roughnessenables the oxide semiconductor film to have stable and highcrystallinity.

Further, an oxide insulating film may be formed between the gateelectrode layer and the aluminum oxide film in the above-describedstructure. Further, an insulating layer with a sidewall structurecovering the side surface of the gate electrode layer may be formedbefore formation of the aluminum oxide film.

Further, heat treatment for releasing hydrogen or moisture may beperformed on the first crystalline oxide semiconductor film in theabove-described structure.

A transistor using such a highly purified crystalline oxidesemiconductor film containing excess oxygen which repairs an oxygenvacancy has less change in electric characteristics and thus iselectrically stable. Accordingly, a highly reliable semiconductor deviceusing an oxide semiconductor having stable electric characteristics canbe provided.

The aluminum oxide film is provided over the crystalline oxidesemiconductor film so that excess oxygen contained in the oxidesemiconductor film is not released by the heat treatment, generation andincrease of a defect in the crystalline oxide semiconductor and in anyinterface between the crystalline oxide semiconductor film and theoverlying or underlying layer in contact with the crystalline oxidesemiconductor film can be prevented. That is, excess oxygen contained inthe crystalline oxide semiconductor film acts to fill an oxygen-vacancydefect, so that a highly reliable semiconductor device having stableelectric characteristics can be provided.

Accordingly, in accordance with one embodiment of the present invention,a transistor having stable electric characteristics can be manufactured.

Further, in accordance with one embodiment of the present invention, ahighly reliable semiconductor device having favorable electriccharacteristics can be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1F show one embodiment of a semiconductor device and amethod for manufacturing the semiconductor device.

FIGS. 2A to 2F show one embodiment of a semiconductor device and amethod for manufacturing the semiconductor device.

FIGS. 3A to 3E show one embodiment of a semiconductor device and amethod for manufacturing the semiconductor device.

FIGS. 4A to 4E show one embodiment of a semiconductor device and amethod for manufacturing the semiconductor device.

FIGS. 5A to 5F show one embodiment of a method for manufacturing asemiconductor device.

FIGS. 6A to 6C show one embodiment of a method for manufacturing asemiconductor device.

FIGS. 7A to 7C show embodiments of a semiconductor device.

FIGS. 8A to 8C show embodiments of a semiconductor device.

FIG. 9 shows SIMS measurement results of Example Sample B.

FIG. 10 shows SIMS measurement results of Comparison Example Sample B.

FIG. 11 shows SIMS measurement results of Comparison Example Sample C.

FIG. 12A shows TDS measurement results of Comparison Example Samples A1and A2; FIG. 12B shows TDS measurement results of Example Samples A1 andA2.

FIG. 13A shows an XRD measurement result of Example Sample D1; FIG. 13Bshows an XRD measurement result of Example Sample D2; FIG. 13C shows anXRD measurement result of Example Sample D3.

FIGS. 14A and 14B are TEM images of Example Sample D1.

FIGS. 15A and 15B are TEM images of Example Sample D2.

FIGS. 16A and 16B are TEM images of Example Sample D3.

FIGS. 17A and 17B are TEM images of Comparison Example Sample D1.

FIGS. 18A and 18B are TEM images of Comparison Example Sample D2.

FIGS. 19A to 19C show one embodiment of a semiconductor device.

FIGS. 20A to 20C show embodiments of a semiconductor device.

FIGS. 21A and 21B show embodiments of a semiconductor device.

FIGS. 22A and 22B show one embodiment of a semiconductor device.

FIGS. 23A to 23F illustrate electronic devices.

FIG. 24 shows SIMS measurement results of Example Samples D2 and D3.

FIGS. 25A to 25C show structures of oxide materials according to oneembodiment of the present invention.

FIG. 26A to 26C show a structure of an oxide material according to oneembodiment of the present invention.

FIGS. 27A to 27C show a structure of an oxide material according to oneembodiment of the present invention.

FIGS. 28A to 28C show a structure of an oxide material according to oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the invention disclosed in thisspecification are described with reference to the accompanying drawings.The invention disclosed in this specification is not limited to thefollowing description, and it will be easily understood by those skilledin the art that modes and details thereof can be variously changedwithout departing from the spirit and the scope of the presentinvention. Therefore, the invention disclosed in this specification isnot construed as being limited to the description of the followingembodiments. The ordinal numbers such as “first” and “second” are usedfor convenience and do not denote the order of steps or the stackingorder of layers. In addition, the ordinal numbers in this specificationdo not denote particular names which specify the present invention.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device and amethod for manufacturing the semiconductor device is described withreference to FIGS. 1A to 1F. In this embodiment, a transistor includingan oxide semiconductor film is described as an example of thesemiconductor device.

A structure of the transistor is not particularly limited as long as itis a top-gate structure; for example, a staggered or a planar type canbe employed. Further, the transistor may have a single gate structureincluding one channel formation region, a double gate structureincluding two channel formation regions, or a triple gate structureincluding three channel formation regions. Alternatively, the transistormay have a dual gate structure including two gate electrode layerspositioned over and below a channel region each with a gate insulatinglayer provided therebetween.

As shown in FIG. 1F, a transistor 440 includes over a substrate 400provided with an insulating layer 436 as an insulating surface, a sourceelectrode layer 405 a, a drain electrode layer 405 b, a crystallineoxide semiconductor film 403, a gate insulating layer 402, and a gateelectrode layer 401. An insulating layer 407 is formed over thetransistor 440.

The insulating layer 407 has either a single-layer structure or astacked-layer structure, and includes an aluminum oxide film. In thisembodiment, an aluminum oxide film is used as the insulating layer 407.

The crystalline oxide semiconductor film 403 is an oxide semiconductorfilm which has an a-b plane substantially parallel to a top surface ofthe crystalline oxide semiconductor film and includes a crystal having ac-axis which is substantially perpendicular to the top surface.Furthermore, the crystalline oxide semiconductor film 403 has neither asingle crystal structure nor an amorphous structure and is a crystallineoxide semiconductor having c-axis alignment (CAAC-OS). The crystallineoxide semiconductor film enables electric characteristics of thetransistor due to irradiation with visible light or ultraviolet light tobe further suppressed to form a highly reliable semiconductor device.

FIGS. 1A to 1F illustrate an example of a method for manufacturing thetransistor 440.

First, the insulating layer 436 is formed over the substrate 400 havingan insulating surface.

There is no particular limitation on a substrate that can be used as thesubstrate 400 having an insulating surface as long as it has heatresistance enough to withstand heat treatment performed later. Forexample, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, or the like, a ceramic substrate, a quartzsubstrate, or a sapphire substrate can be used. A single crystalsemiconductor substrate or a polycrystalline semiconductor substratecomprised of silicon, silicon carbide, or the like; a compoundsemiconductor substrate of silicon germanium or the like; an SOIsubstrate; or the like can be used as the substrate 400, or such asubstrate provided with a semiconductor element can also be used as thesubstrate 400.

The semiconductor device may be manufactured using a flexible substrateas the substrate 400. In order to manufacture a flexible semiconductordevice, the transistor 440 including the crystalline oxide semiconductorfilm 403 may be directly formed over a flexible substrate; oralternatively, the transistor 440 including the crystalline oxidesemiconductor film 403 may be formed over a substrate, and then, thetransistor 440 may be separated and transferred to a flexible substrate.In order to separate the transistor from the substrate and transfer tothe flexible substrate, a separation layer may be provided between thesubstrate and the transistor including the oxide semiconductor film.

The insulating layer 436 can be formed by a plasma enhanced CVD method,a sputtering method, or the like using silicon oxide, siliconoxynitride, aluminum oxide, aluminum oxynitride, hafnium oxide, galliumoxide, silicon nitride, silicon nitride oxide, aluminum nitride,aluminum nitride oxide, or a mixed material thereof.

The insulating layer 436 has either a single-layer structure or astacked-layer structure; an oxide insulating layer is preferably used asthe film to be in contact with the crystalline oxide semiconductor film403. A silicon oxide film is formed by a sputtering method as theinsulating layer 436 in this embodiment.

Next, a crystalline oxide semiconductor film 444 is formed over theinsulating layer 436 (see FIG. 1A).

The insulating layer 436, which is in contact with the crystalline oxidesemiconductor film 444, preferably contains oxygen which exceeds atleast the stoichiometric composition ratio in the film (the bulk). Forexample, in the case where a silicon oxide film is used as theinsulating layer 436, the composition formula is SiO_(2+α) (α>0). Byusing such a film as the insulating layer 436 described above, oxygencan be supplied to the crystalline oxide semiconductor film 444, leadingto favorable characteristics. Oxygen supplied to the crystalline oxidesemiconductor film 444 can repair oxygen vacancies in the film.

For example, an insulating layer containing a large amount of (an excessof) oxygen, which is a supply source of oxygen, may be provided as theinsulating layer 436 so as to be in contact with the crystalline oxidesemiconductor film 444, whereby oxygen can be supplied from theinsulating layer 436 to the crystalline oxide semiconductor film 444.Heat treatment may be performed in the state where the crystalline oxidesemiconductor film 444 and the insulating layer 436 are in contact witheach other at least partly, so that oxygen is supplied to thecrystalline oxide semiconductor film 444.

Further, a top surface of the insulating layer 436 in a region which isin contact with the crystalline oxide semiconductor film 444 preferablyhas less roughness. Specifically, the average surface roughness of thetop surface is preferably greater than or equal to 0.05 nm and less than0.5 nm (or greater than or equal to 0.1 nm and less than 0.5 nm). Such atop surface of the insulating layer with less surface roughness enablesthe crystalline oxide semiconductor film 444 to have stable and highcrystallinity.

In this specification and the like, average surface roughness (R_(a)) isobtained by three-dimension expansion of the center line averageroughness (R_(a)) which is defined by JISB0601:2001 (ISO 4287:1997) sothat R_(a) can be applied to a measurement surface, and is an averagevalue of the absolute values of deviations from a reference surface to aspecific surface.

The center line average roughness (R_(a)) is expressed by the followingformula (1), where a portion by a measurement length L is picked up froma roughness curve in the direction of the center line of the roughnesscurve, the direction of a center line of the roughness curve of thepicked portion is an X-axis, the direction of longitudinal magnification(direction perpendicular to the X-axis) is a Y-axis, and the roughnesscurve is expressed by Y=F(X).

[FORMULA  1]                                    $\begin{matrix}{R_{a} = \left. {\frac{1}{L}\int_{0}^{L}} \middle| {F(X)} \middle| \ {d\; X} \right.} & (1)\end{matrix}$

Where the measurement surface which is a surface represented bymeasurement data is expressed by Z=F(X,Y), the average surface roughness(R_(a)) is an average value of the absolute values of deviations fromthe reference surface to the specific surface and is expressed by thefollowing formula (2).

[FORMULA  2]                                    $\begin{matrix}{R_{a} = \left. {\frac{1}{S_{0}}{\int_{Y_{1}}^{Y_{2}}\int_{X_{1}}^{X_{2}}}} \middle| {{F\left( {X,Y} \right)} - Z_{0}} \middle| \ {d\; X\ d\; Y} \right.} & (2)\end{matrix}$

Here, the specific surface is a surface which is a target of roughnessmeasurement, and is a rectangular region which is surrounded by fourpoints represented by the coordinates (X₁,Y₁), (X₁,Y₂), (X₂,Y₁), and(X₂,Y₂), and S₀ denotes the area of the specific surface when assumed tobe flat ideally.

In addition, the reference surface refers to a surface parallel to theX-Y plane at the average height of the specific surface. In short, wherethe average value of the height of the specific surface is Z₀, theheight of the reference surface is also denoted by Z₀.

Therefore, planarizing treatment may be performed on the region of theinsulating layer 436 which is in contact with the crystalline oxidesemiconductor film 444. As the planarizing treatment, polishingtreatment (e.g., chemical mechanical polishing (CMP)), dry-etchingtreatment, or plasma treatment can be used, though there is noparticular limitation on the planarizing treatment.

As the plasma treatment, a reverse sputtering in which an argon gas isintroduced and plasma is produced can be performed. The reversesputtering is a method in which voltage is applied to a substrate sidewith use of an RF power source in an argon atmosphere and plasma isformed in the vicinity of the substrate so that a substrate surface ismodified. Instead of the argon atmosphere, a nitrogen atmosphere, ahelium atmosphere, an oxygen atmosphere, or the like may be used. Thereverse sputtering can remove particle substances (also referred to asparticles or dust) attached to the top surface of the insulating layer436.

As the planarizing treatment, polishing treatment, dry-etchingtreatment, or plasma treatment may be performed plural times and/or incombination. Further, the order of steps of such a combination is notparticularly limited and may be set as appropriate in accordance withroughness of the top surface of the insulating layer 436.

In order that hydrogen or water will not enter the crystalline oxidesemiconductor film 444 as much as possible in the formation step of thecrystalline oxide semiconductor film 444, it is preferable to heat thesubstrate provided with the insulating layer 436 in a preheating chamberin a sputtering apparatus as a pretreatment for formation of thecrystalline oxide semiconductor film 444 so that impurities such ashydrogen and moisture adsorbed to the substrate and the insulating layer436 are eliminated and evacuated. As an exhaustion unit provided in thepreheating chamber, a cryopump is preferable.

The crystalline oxide semiconductor film 444 is an oxide semiconductorfilm having a crystallized portion; CAAC-OS (c-axis aligned crystallineoxide semiconductor) is used in this embodiment. The crystalline oxidesemiconductor film 444 contains a crystal having a c-axis which issubstantially perpendicular to a top surface of the crystalline oxidesemiconductor film 444.

CAAC-OS is an oxide semiconductor containing a crystal with c-axisalignment which has a triangular or hexagonal atomic arrangement whenseen from the direction of the a-b plane, the top surface, or theinterface and in which metal atoms are arranged in a layered manner, ormetal atoms and oxygen atoms are arranged in a layered manner along thec-axis, and the direction of the a-axis or the b-axis is varied in thea-b plane (or the top surface or the interface), that is, which rotatesaround the c-axis.

In a broad sense, CAAC-OS means a non-single-crystal material includinga phase which has a triangular, hexagonal, regular triangular, orregular hexagonal atomic arrangement when seen from the directionperpendicular to the a-b plane and in which metal atoms are arranged ina layered manner, or metal atoms and oxygen atoms are arranged in alayered manner when seen from the direction perpendicular to the c-axisdirection.

The CAAC-OS is not a single crystal, but is not only in an amorphousstate. The CAAC-OS includes a crystallized portion (crystallineportion), and a boundary between one crystalline portion and anothercrystalline portion is not clear in some positions.

Nitrogen may be substituted for part of oxygen which constitutes part ofthe CAAC-OS. The c-axes of individual crystalline portions included inthe CAAC-OS may be aligned in one direction (e.g., the directionperpendicular to a surface of a substrate over which the CAAC-OS isformed, or a top surface, a film surface, and an interface of theCAAC-OS, or the like). Alternatively, the normals of the a-b planes ofthe individual crystalline portions included in the CAAC-OS may bealigned in one direction (e.g., a direction perpendicular to thesubstrate surface or the surface, film surface, interface, or the likeof the CAAC-OS).

First, CAAC-OS is described in detail using FIGS. 25A to 25C and FIGS.26A to 26C. In FIGS. 25A to 25C and FIGS. 26A and 26B, the upwarddirection corresponds to the c-axis direction and a plane perpendicularto the c-axis direction corresponds to the a-b plane, unless otherwisespecified. The simple expressions of an “upper half” and a “lower half”refer to an upper half above the a-b plane and a lower half below thea-b plane (an upper half and a lower half with respect to the a-bplane), respectively. Furthermore, in FIGS. 25A to 25C, O surrounded bya circle represents tetracoodianate O and O surrounded by a doublecircle represents tricoodenate O.

FIG. 25A illustrates a structure having one hexacoordinate metal atomM_1 and six tetracoordinate oxygen atoms O proximate to the metal atomM_1. Such a structure in which only one metal atom and proximate oxygenatoms to the metal atom are illustrated is called a subunit herein. Thestructure in FIG. 25A is an octahedral structure, but is illustrated asa planar structure for simplicity. Three tetracoordinate O exist in eachof the upper half and the lower half in FIG. 25A.

FIG. 25B illustrates a structure having one pentacoordinate metal atomM_2, three tricoordinate oxygen atoms (hereinafter referred to astricoordinate O) proximate to the metal atom M_2, and twotetracoordinate O proximate to the metal atom M_2. All the tricoordinateO exist on the a-b plane. One tetracoordinate O exists in each of theupper half and the lower half in FIG. 25B.

FIG. 25C illustrates a structure having one tetracoordinate metal atomM_3 and four tetracoordinate O proximate to the metal atom M_3. Onetetracoordinate O exists in the upper half and three tetracoordinate Oexist in the lower half in FIG. 25C. Alternatively, threetetracoordinate O atoms may exist in the upper half and onetetracoordinate O atom may exist in the lower half in FIG. 25C.

Such a metal atom whose coordination number is 4, 5, or 6 is bonded toanother metal atom through tetracoordinate O. Specifically, a metal atomis bonded to another metal atom through four tetracoordinate O in total.For example, in the case where the hexacoordinate metal atom M_1 isbonded through three tetracoordinate O in the upper half, it is bondedto the pentacoordinate metal atom M_2 through tetracoordinate O in theupper half with respect to the pentacoordinate metal atom M_2, thepentacoordinate metal atom M_2 through tetracoordinate O in the lowerhalf with respect to the pentacoordinate metal atom M_2, or thetetracoordinate metal atom M_3 through the tetracoordinate O in theupper half with respect to the tetracoordinate metal atom M_3.

Such a metal atom whose coordination number is 4, 5, or 6 is bonded toanother metal atom through tetracoordinate O. Besides, subunits arebonded to each other so that the total electric charge in the layerstructure is 0 to constitute one group.

FIG. 26A illustrates a model of one group which constitutes a layerstructure of an In—Sn—Zn—O system. A structure in which only one metalatom and proximate oxygen atoms to the metal atom are illustrated isreferred to as a subunit, a group of a plurality of subunits is referredto as one group, and one cycle which consists of a plurality of groupsas shown in FIG. 26B is referred to as a unit herein. FIG. 26Cillustrates an atomic arrangement of the layer structure in FIG. 26Bwhen observed from the direction perpendicular to the film top surface,the substrate surface, or the interface.

In FIG. 26A, tricoordinate O is omitted for simplicity, andtetracoordinate O is illustrated by circling the number thereof. Forexample, three tetracoordinate O existing in each of the upper half andthe lower half with respect to a Sn atom are denoted by circled 3.Similarly, in FIG. 26A, one tetracoordinate O existing in each of theupper half and the lower half with respect to an In atom is denoted bycircled 1. FIG. 26A also illustrates a Zn atom proximate to onetetracoordinate O in the lower half and three tetracoordinate O in theupper half, and a Zn atom proximate to one tetracoordinate O in theupper half and three tetracoordinate O in the lower half.

In FIG. 26A, one group which constitutes the layer structure of anIn—Sn—Zn—O system is a structure in which the following metal atoms arebonded to each other in this order from the top: the Sn atom proximateto three tetracoordinate O in each of the upper half and the lower half;the In atom proximate to one tetracoordinate O in each of the upper halfand the lower half; the Zn atom proximate to three tetracoordinate O inthe upper half; the In atom proximate to three tetracoordinate O in eachof the upper half and the lower half, which is bonded to the Zn atomthrough one tetracoordinate O in the lower half with respect to the Znatom; the Zn atom proximate to one tetracoordinate O in the upper half;the Zn atom which is bonded to the Zn atom through three tetracoordinateO in the lower half with respect to the Zn atom; and Sn atom which isbonded to the Zn atom through one tetracoordinate O in the lower halfwith respect to the Zn atom. A plurality of groups is bonded toconstitute one cycle, one unit.

Here, electric charge for one bond of tricoordinate O and electriccharge for one bond of tetracoordinate O can be assumed to be −0.667 and−0.5, respectively. For example, electric charge of a (hexacoordinate orpentacoordinate) In atom, electric charge of a (tetracoordinate) Znatom, and electric charge of a (pentacoordinate or hexacoordinate) Snatom are +3, +2, and +4, respectively. Therefore, electric charge of asubunit of a Sn atom is +1. Thus, electric charge of −1, which cancels+1, is needed to form a layer structure including a Sn atom. As such astructure having electric charge of −1, a structure in which twosubunits of Zn atoms are bonded as shown in FIG. 26A can be given. Forexample, two bonded subunits of Zn atoms with respect to one subunit ofa Sn atom enables electric charge to be canceled, whereby the totalelectric charge in the layer structure can result in 0.

An In atom can have either 5 ligands or 6 ligands. With a plurality ofcycles each illustrated in FIG. 26B, an In—Sn—Zn—O-based crystal(In₂SnZn₃O₈) can be formed. A layer structure of the In—Sn—Zn—O systemcan be expressed by a composition formula, In₂SnZn₂O₇(ZnO)_(m) (m is 0or a natural number). As larger m is, the crystallinity of theIn—Sn—Zn—O-based crystal improves, which is preferable.

The above can be applied also in the case where the following is used: afour-component metal oxide such as an In—Sn—Ga—Zn—O-based oxidesemiconductor; a three-component metal oxide such as an In—Ga—Zn—O-basedoxide semiconductor (also referred to as IGZO), an In—Al—Zn—O-basedoxide semiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, anAl—Ga—Zn—O-based oxide semiconductor, or a Sn—Al—Zn—O-based oxidesemiconductor; a two-component metal oxide such as an In—Zn—O-basedoxide semiconductor, a Sn—Zn—O-based oxide semiconductor, anAl—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxide semiconductor,a Sn—Mg—O-based oxide semiconductor, an In—Mg—O-based oxidesemiconductor, or an In—Ga—O-based oxide semiconductor; or asingle-component metal oxide such as an In—O-based oxide semiconductor,a Sn—O-based oxide semiconductor, or a Zn—O-based oxide semiconductor.

For example, FIG. 27A illustrates a model of a first group as an exampleof a structure of a layer structure of an In—Ga—Zn—O system.

In FIG. 27A, the first group which constitutes the layer structure of anIn—Ga—Zn—O system is a structure in which the following metal atoms arebonded to each other in this order from the top: an In atom proximate tothree tetracoordinate O in each of the upper half and the lower half; aGa atom proximate to one tetracoordinate O in the upper half; a Zn atomproximate to one tetracoordinate O in each of the upper half and thelower half, which is bonded to the Ga atom through three tetracoordinateO in the lower half with respect to the Ga atom; and an In atomproximate to three tetracoordinate O in each of the upper half and thelower half, which is bonded to the Zn atom through one tetracoordinate Oin the lower half with respect to the Zn atom. A plurality of (3 in thisembodiment) first groups is bonded to constitute one cycle, one unit.

One cycle which consists of a plurality of first groups is illustratedin FIG. 27B. FIG. 27C illustrates an atomic arrangement of the layerstructure in FIG. 27B when observed from the direction perpendicular tothe film top surface, the substrate surface, or the interface.

The group which constitutes the layer structure of an In—Ga—Zn—O systemis not limited to the first group shown in FIG. 27A, and instead,another combination of subunits can be used. For example, a second groupwhich constitutes another In—Ga—Zn—O system is illustrated in FIG. 28A,and one cycle which consists of a plurality of second groups isillustrated in FIG. 28B. FIG. 28C illustrates an atomic arrangement ofthe layer structure in FIG. 28B when observed from the directionperpendicular to the film top surface, the substrate surface, or theinterface.

FIG. 28A illustrates a model of the second group as another example of astructure of a layer structure of an In—Ga—Zn—O system.

In FIG. 28A, the second group which constitutes the layer structure ofan In—Ga—Zn—O system is a structure in which the following metal atomsare bonded to each other in this order from the top: an In atomproximate to three tetracoordinate O in each of the upper half and thelower half; a Ga atom proximate to one tetracoordinate O in each of theupper half and the lower half; a Zn atom proximate to threetetracoordinate O in the upper half; and an In atom proximate to threetetracoordinate O in each of the upper half and the lower half, which isbonded to the Zn atom through one tetracoordinate O in the lower halfwith respect to the Zn atom. A plurality of (3 in this embodiment)second groups is bonded to constitute one cycle, one unit.

Electric charge of a (hexacoordinate or pentacoordinate) In atom,electric charge of a (tetracoordinate) Zn atom, and electric charge of a(hexacoordinate) Ga atom are +3, +2, and +3, respectively. Therefore,electric charge of a subunit of an In atom, a Zn atom, and a Ga atomresults in 0. Thus, the total electric charge of a layer structure whichconsists of a combination of the subunits is always 0.

The group which constitutes the layer structure of an In—Ga—Zn—O systemis not limited to the first group and the second group; a variety ofatoms can be used in combination to constitute the group. For example,any combination can be used as long as it has c-axis alignment and has atriangular, hexagonal, regular triangular, or regular hexagonal atomicarrangement when seen from the direction of the a-b plane, the topsurface, or the interface, and metal atoms are arranged in a layeredmanner, or metal atoms and oxygen atoms are arranged in a layered mannerin the c-axis direction. Further, one unit is not limited to 3 firstgroups or 3 second groups but can consist of any other combination.

There are three methods for obtaining a crystalline oxide semiconductorhaving c-axis alignment. First is a method in which an oxidesemiconductor film is deposited at a temperature(s) higher than or equalto 200° C. and lower than or equal to 500° C. such that the c-axis issubstantially perpendicular to the top surface. Second is a method inwhich an oxide semiconductor film is deposited thin, and is subjected toheat treatment at a temperature(s) higher than or equal to 200° C. andlower than or equal to 700° C., so that the c-axis is substantiallyperpendicular to the top surface. Third is a method in which afirst-layer oxide semiconductor film is deposited thin, and is subjectedto heat treatment at a temperature(s) higher than or equal to 200° C.and lower than or equal to 700° C., and a second-layer oxidesemiconductor film is deposited thereover, so that the c-axis issubstantially perpendicular to the top surface.

In this embodiment, an oxide semiconductor film is deposited at atemperature(s) higher than or equal to 200° C. and lower than or equalto 500° C., thereby forming the crystalline oxide semiconductor film 444having a c-axis alignment substantially perpendicular to a top surfaceof the crystalline oxide semiconductor film 444. For example, thecrystalline oxide semiconductor film 444 having a c-axis alignmentsubstantially perpendicular to the top surface is deposited by asputtering method at a substrate temperature of 400° C.

The crystalline oxide semiconductor film 444 is CAAC-OS, which enables achange of electric characteristics of the transistor due to irradiationwith visible light or ultraviolet light to be further suppressed, sothat a highly reliable semiconductor device can be provided.

The crystalline oxide semiconductor film 444 has a thickness greaterthan or equal to 1 nm and less than or equal to 200 nm (preferablygreater than or equal to 5 nm and less than or equal to 30 nm) and canbe formed by a sputtering method, a molecular beam epitaxy (MBE) method,a CVD method, a pulse laser deposition method, an atomic layerdeposition (ALD) method, or the like as appropriate. The crystallineoxide semiconductor film 444 may be formed using a sputtering apparatuswhich performs deposition with top surfaces of a plurality of substratesset substantially perpendicular to a top surface of a sputtering target,which is a so-called columnar plasma (CP) sputtering system. By any ofthe methods, crystal growth proceeds in the direction perpendicular toroughness of the top surface of the oxide semiconductor film, so that acrystalline oxide semiconductor having c-axis alignment can be obtained.

As a material of the crystalline oxide semiconductor film 444, at leastone element selected from In, Ga, Sn, and Zn is contained. For example,the following can be used: a four-component metal oxide such as anIn—Sn—Ga—Zn—O-based oxide semiconductor; a three-component metal oxidesuch as an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-basedoxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, aSn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxidesemiconductor, a Sn—Al—Zn—O-based oxide semiconductor, or aHf—In—Zn—O-based oxide semiconductor; a two-component metal oxide suchas an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxidesemiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-basedoxide semiconductor, a Sn—Mg—O-based oxide semiconductor, anIn—Mg—O-based oxide semiconductor, or an In—Ga—O-based oxidesemiconductor; a single-component metal oxide such as an In—O-basedoxide semiconductor, a Sn—O-based oxide semiconductor, or a Zn—O-basedoxide semiconductor; or the like. In addition, any of the above oxidesemiconductors may contain an element other than In, Ga, Sn, and Zn, forexample, SiO₂.

For example, the In—Ga—Zn—O-based oxide semiconductor means an oxidesemiconductor containing indium (In), gallium (Ga), and zinc (Zn), andthere is no limitation on the composition ratio thereof.

Further, for the crystalline oxide semiconductor film 444, a thin filmrepresented by the chemical formula, InMO₃(ZnO)_(m) (m>0), can be used.Here, M represents one or more metal elements selected from Zn, Ga, Al,Mn, and Co. For example, M can be Ga, Ga and Al, Ga and Mn, Ga and Co,or the like.

In the case where an In—Sn—Zn—O-based oxide semiconductor is used as anoxide semiconductor, a target whose composition ratio is In:Sn:Zn=1:2:2,In:Sn:Zn=2:1:3, In:Sn:Zn=1:1:1, or the like in an atomic ratio, can beused.

In the case where an In—Zn—O-based material is used as an oxidesemiconductor, a target therefor has a composition ratio of In:Zn=50:1to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio),preferably In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2in a molar ratio), further preferably In:Zn=15:1 to 1.5:1 in an atomicratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, in a targetused for deposition of an In—Zn—O-based oxide semiconductor where anatomic ratio of In:Zn:O=X:Y:Z, the relation of Z>1.5X+Y is satisfied.

The crystalline oxide semiconductor film 444 is preferably depositedunder a condition such that much oxygen is contained (for example, by asputtering method in an atmosphere of 100% oxygen) so as to be a filmcontaining much oxygen (preferably having a region containing an excessof oxygen as compared to the stoichiometric composition ratio of theoxide semiconductor in a crystalline state).

Further, heat treatment may be performed on the crystalline oxidesemiconductor film 444 in order to remove excess hydrogen (includingwater and a hydroxyl group) (to perform dehydration or dehydrogenation).The temperature of the heat treatment is higher than or equal to 300° C.and lower than or equal to 700° C., or lower than the strain point of asubstrate. The heat treatment can be performed under reduced pressure, anitrogen atmosphere, or the like. For example, the substrate isintroduced into an electric furnace which is a kind of heat treatmentapparatus, and heat treatment is performed on the oxide semiconductorfilm at 450° C. for 1 hour in a nitrogen atmosphere.

The heat treatment apparatus is not limited to an electric furnace, andinstead, any device for heating an object by heat conduction or heatradiation from a heating element such as a resistance heating elementmay be used. For example, an RTA (rapid thermal anneal) apparatus suchas a GRTA (gas rapid thermal anneal) apparatus or an LRTA (lamp rapidthermal anneal) apparatus can be used. The LRTA apparatus is anapparatus for heating an object by radiation of light (electromagneticwave) emitted from a lamp such as a halogen lamp, a metal halide lamp, axenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or ahigh pressure mercury lamp. The GRTA apparatus is an apparatus for heattreatment using a high-temperature gas. As the high-temperature gas, aninert gas which does not react with an object by heat treatment, such asnitrogen or a rare gas like argon, is used.

For example, as the heat treatment, GRTA may be performed as follows:the substrate is put in an inert gas heated at a high temperature ofbetween 650° C. and 700° C., and is heated for several minutes, and isthen taken out of the inert gas.

Such heat treatment for dehydration or dehydrogenation may be performedin the manufacturing process of the transistor 440 anytime afterformation of the crystalline oxide semiconductor film 444 beforeaddition of oxygen into a crystalline oxide semiconductor film 413.

The heat treatment for dehydration or dehydrogenation is preferablyperformed before the crystalline oxide semiconductor film 444 isprocessed into an island shape to be the crystalline oxide semiconductorfilm 413, whereby oxygen contained in the insulating layer 436 can beprevented from being released by the heat treatment.

In the heat treatment, it is preferable that water, hydrogen, or thelike is not contained in nitrogen or a rare gas such as helium, neon, orargon; or the purity of nitrogen or the rare gas such as helium, neon,or argon which is introduced into the heat treatment apparatus ispreferably 6N (99.9999%) or higher, further preferably 7N (99.99999%) orhigher (that is, the impurity concentration is preferably 1 ppm orlower, further preferably 0.1 ppm or lower).

In addition, after the crystalline oxide semiconductor film 444 isheated by the heat treatment, a high-purity oxygen gas, a high-purityN₂O gas, or ultra dry air (the moisture amount is less than or equal to20 ppm (−55° C. by conversion into a dew point), preferably less than orequal to 1 ppm, further preferably less than or equal to 10 ppb,according to a dew point meter of a cavity ring down laser spectroscopy(CRDS) system) may be introduced into the same furnace. It is preferablethat water, hydrogen, or the like is not contained in the oxygen gas orthe N₂O gas; or the purity of the oxygen gas or the N₂O gas which isintroduced into the heat treatment apparatus is preferably 6N or higher,further preferably 7N or higher (i.e., the impurity concentration in theoxygen gas or the N₂O gas is preferably 1 ppm or lower, furtherpreferably 0.1 ppm or lower). The oxygen gas or the N₂O gas acts tosupply oxygen that is a main component of the crystalline oxidesemiconductor and that is reduced by the step for removing an impurityfor dehydration or dehydrogenation, thereby making the crystalline oxidesemiconductor film 444 a highly purified, electrically i-type(intrinsic) oxide semiconductor.

The crystalline oxide semiconductor film 444 may be processed into anisland shape or may not be processed to remain the film shape. Further,an element isolation region formed of an insulating layer for isolatingthe crystalline oxide semiconductor film per element may be provided.

In this embodiment, the crystalline oxide semiconductor film 444 isprocessed into the island-shaped crystalline oxide semiconductor film413 by a photolithography process. A resist mask used for forming theisland-shaped crystalline oxide semiconductor film 413 may be formed byan inkjet method. Formation of the resist mask by an inkjet methodinvolves no photomask; thus, manufacturing costs can be reduced.

One or both of dry etching and wet etching may be used for etching ofthe crystalline oxide semiconductor film 444. As an etchant used for wetetching for the crystalline oxide semiconductor film 444, for example, amixed solution of phosphoric acid, acetic acid, and nitric acid, or thelike can be used. Further, ITO07N (produced by KANTO CHEMICAL CO., INC.)may also be used.

Next, a gate insulating layer 442 covering the crystalline oxidesemiconductor film 413 is formed (see FIG. 1B).

To improve the coverage with the gate insulating layer 442 over thecrystalline oxide semiconductor film 413, the above-describedplanarizing treatment may be performed also on a top surface of thecrystalline oxide semiconductor film 413. It is preferable that theplanarity of the top surface of the crystalline oxide semiconductor film413 is high particularly in the case where a thin insulating film isused as the gate insulating layer 442.

The gate insulating layer 442 has a thickness greater than or equal to 1nm and less than or equal to 100 nm and can be formed by a sputteringmethod, an MBE method, a CVD method, a pulse laser deposition method, anALD method, or the like as appropriate. The gate insulating layer 442may also be formed using a sputtering apparatus which performs filmdeposition with surfaces of a plurality of substrates set substantiallyperpendicular to a top surface of a sputtering target, which is aso-called columnar plasma (CP) sputtering system.

The gate insulating layer 442 can be formed using a silicon oxide film,a gallium oxide film, an aluminum oxide film, a silicon nitride film, asilicon oxynitride film, an aluminum oxynitride film, or a siliconnitride oxide film. It is preferable that the gate insulating layer 442include oxygen in a portion which is in contact with the crystallineoxide semiconductor film 413. In particular, the gate insulating layer442 preferably contains an excess amount of oxygen which exceeds atleast the stoichiometry composition ratio in the film (bulk); forexample, in the case where a silicon oxide film is used as the gateinsulating layer 442, the composition formula is SiO_(2+α) (α>0). Inthis embodiment, a silicon oxide film of SiO_(2+α) (α>0) is used as thegate insulating layer 442. By using the silicon oxide film as the gateinsulating layer 442, oxygen can be supplied to the crystalline oxidesemiconductor film 413 and favorable characteristics can be obtained.Further, the gate insulating layer 442 is preferably formed inconsideration of the size of a transistor and the step coverage with thegate insulating layer 442.

The gate insulating layer 442 can be formed using a high-k material suchas hafnium oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0,y>0)), hafnium silicate to which nitrogen is added (HfSiO_(x)N_(y) (x>0,y>0)), hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)), or lanthanum oxide,whereby gate leakage current can be reduced. Further, the gateinsulating layer 442 has either a single-layer structure or astacked-layer structure.

Next, oxygen 431 (including at least one of an oxygen radical, an oxygenatom, and an oxygen ion) is added into the crystalline oxidesemiconductor film 413; thus, oxygen is supplied to the crystallineoxide semiconductor film 413. Oxygen can be added by an ion implantationmethod, an ion doping method, a plasma immersion ion implantationmethod, plasma treatment, or the like.

The step of adding oxygen in the manufacturing process of the transistor440 in this embodiment is performed after formation of the crystallineoxide semiconductor film 444 before formation of an aluminum oxide filmas the insulating layer 407. The above-described heat treatment fordehydration or dehydrogenation is performed before the step of addingoxygen. In the step of adding oxygen, oxygen may be directly added intothe crystalline oxide semiconductor film or added into the crystallineoxide semiconductor film through another film such as the gateinsulating layer or an insulating layer. An ion implantation method, anion doping method, a plasma immersion ion implantation method, or thelike may be employed in the case where oxygen is added into acrystalline oxide semiconductor film through another film, whereasplasma treatment or the like can also be employed in the case whereoxygen is directly added into a crystalline oxide semiconductor filmwhich is bare (for example, subsequently to formation of the crystallineoxide semiconductor film 444 or subsequently to formation of thecrystalline oxide semiconductor film 413).

In this embodiment, the oxygen 431 is added into the crystalline oxidesemiconductor film 413 through the gate insulating layer 442 by an ionimplantation method. The addition of the oxygen 431 amorphizes at leastpart of the crystalline oxide semiconductor film 413, so that anamorphous oxide semiconductor film 443 having a region where the amountof oxygen is excess as compared to the stoichiometric composition ratioof the oxide semiconductor in a crystalline state is formed (see FIG.1C).

For example, it is preferable that the concentration of oxygen in theamorphous oxide semiconductor film 443, which is added by the additionof the oxygen 431, is greater than or equal to 1×10¹⁸/cm³ and less thanor equal to 3×10²¹/cm³. Such an oxygen excess region exists in at leastpart (including its interface) of the amorphous oxide semiconductor film443. Thus, with the addition of the oxygen 431, oxygen is contained inat least one of the interface between the insulating layer 436 and theamorphous oxide semiconductor film 443, the amorphous oxidesemiconductor film 443, and the interface between the amorphous oxidesemiconductor film 443 and the gate insulating layer 442.

The amorphous oxide semiconductor film 443 has the region containing anexcess of oxygen as compared to the stoichiometric composition ratio ofthe oxide semiconductor in a crystalline state. In that case, the oxygencontent is greater than that in the stoichiometric oxide semiconductoror higher than that in the single crystal state. In some cases, oxygenexists between lattices of the oxide semiconductor. The composition ofsuch an oxide semiconductor can be expressed by InGaZn_(m)O_(m+3x)(x>1). For example, supposing that m=1, the value of 1+3x inInGaZnO_(1+3x) (x>1) exceeds 4 in the case where the content of oxygenis excess.

An oxygen vacancy in the amorphous oxide semiconductor film 443 can berepaired by the oxygen 431.

In the oxide semiconductor, oxygen is one of main component materials.Thus, it is difficult to accurately estimate the oxygen concentration ofthe oxide semiconductor film by a method such as secondary ion massspectrometry (SIMS). In other words, it can be said that it is difficultto judge whether oxygen is intentionally added to the oxidesemiconductor film or not.

However, it is known that there exist isotopes of oxygen, such as ¹⁷Oand ¹⁸O, and the proportions of ¹⁷O and ¹⁸O in the total of oxygen atomsin nature are about 0.037% and about 0.204%, respectively. That is tosay, it is possible to measure the concentrations of these isotopes inthe oxide semiconductor film by a method such as SIMS; thus, the oxygenconcentration of the oxide semiconductor film may be able to beestimated more accurately by measuring the concentrations of theseisotopes. Therefore, the concentrations of these isotopes may bemeasured to determine whether oxygen is intentionally added to the oxidesemiconductor film or not.

In this specification, since at least part of a crystal having c-axiswhich is substantially perpendicular to the top surface of thecrystalline oxide semiconductor film 413 is amorphized to lower thecrystallinity by adding the oxygen 431 into the crystalline oxidesemiconductor film 413, the crystalline oxide semiconductor film 413after being added with the oxygen 431 is referred to as the amorphousoxide semiconductor film 443.

A film containing much oxygen may be used as the insulating layer whichis in contact with the crystalline oxide semiconductor film 413 and theoxygen 431 may be directly added into the crystalline oxidesemiconductor film 413; in this manner, a plurality of oxygen supplymethods can be performed. Such a film containing much oxygen is notnecessarily used as the insulating layer which is in contact with thecrystalline oxide semiconductor film 413 in the case where the oxygen431 is directly added into the crystalline oxide semiconductor film 413like this embodiment.

Next, the gate electrode layer 401 is formed over the gate insulatinglayer 442. The gate electrode layer 401 can be formed using a metalmaterial such as molybdenum, titanium, tantalum, tungsten, aluminum,copper, chromium, neodymium, or scandium or an alloy material whichcontains any of these materials as its main component. A semiconductorfilm which is doped with an impurity element such as phosphorus and istypified by a polycrystalline silicon film, or a silicide film of nickelsilicide or the like can also be used as the gate electrode layer 401.The gate electrode layer 401 has either a single-layer structure or astacked-layer structure.

The gate electrode layer 401 can also be formed using a conductivematerial such as indium tin oxide, indium oxide containing tungstenoxide, indium zinc oxide containing tungsten oxide, indium oxidecontaining titanium oxide, indium tin oxide containing titanium oxide,indium zinc oxide, or indium tin oxide to which silicon oxide is added.It is also possible to have a stacked-layer structure formed using theabove conductive material and the above metal material.

As one layer of the gate electrode layer 401 which is in contact withthe gate insulating layer 442, a metal oxide containing nitrogen,specifically, an In—Ga—Zn—O film containing nitrogen, an In—Sn—O filmcontaining nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—Ofilm containing nitrogen, a Sn—O film containing nitrogen, an In—O filmcontaining nitrogen, or a metal nitride (e.g., InN or SnN) film can beused. These films each have a work function of 5 eV or higher,preferably 5.5 eV or higher; thus, any of these films used as the gateelectrode layer enables the threshold voltage of the transistor to bepositive, so that a so-called normally-off switching element can beprovided.

Sidewall insulating layers 412 a and 412 b are formed on the sidesurface of the gate electrode layer 401, and the gate insulating layer402 is formed. The sidewall insulating layers 412 a to 412 d may beformed on the side surface of the gate electrode layer 401 in aself-aligned manner by forming an insulating layer to cover the gateelectrode layer 401 and then processing the insulating layer byanisotropic etching by an RIE (Reactive Ion Etching) method. There is noparticular limitation on the insulating layer; for example, a siliconoxide film with favorable step coverage, which is formed by reactingTEOS (tetraethyl ortho-silicate), silane, or the like with oxygen,nitrous oxide, or the like can be used. The insulating layer can beformed by a thermal CVD method, a plasma enhanced CVD method, anatmospheric pressure CVD method, a bias ECRCVD method, a sputteringmethod, or the like. A silicon oxide film formed by a low temperatureoxidation (LTO) method may also be used.

The gate insulating layer 402 can be formed by etching the gateinsulating layer 442 with use of the gate electrode layer 401 and thesidewall insulating layers 412 a and 412 b as a mask.

In this embodiment, in etching the insulating layer, the insulatinglayer over the gate electrode layer 401 is removed to expose the gateelectrode layer 401; alternatively, the sidewall insulating layers 412 ato 412 d may be formed while the insulating layer directly above thegate electrode layer 401 remains. Further, a protective film may beformed over the gate electrode layer 401 in a later step. By protectingthe gate electrode layer 401 in such a manner, film reduction of thegate electrode layer in the etching process can be prevented. Variousetching methods such as a dry etching method and a wet etching methodmay be used for the etching.

Next, a conductive film for forming a source electrode layer and a drainelectrode layer (including a wiring formed of the same layer) is formedover part of the sidewall insulating layer 412 a, part of the sidewallinsulating layer 412 b, and the amorphous oxide semiconductor film 443.The conductive film is formed using a material that can withstand heattreatment in a later step. As the conductive film used for the sourceelectrode layer and the drain electrode layer, for example, a metal filmcontaining an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W, ametal nitride film containing any of the above elements as its component(a titanium nitride film, a molybdenum nitride film, or a tungstennitride film), or the like can be used. A metal film having a highmelting point of Ti, Mo, W, or the like or a metal nitride film of anyof these elements (such as a titanium nitride film, a molybdenum nitridefilm, or a tungsten nitride film) may be stacked on one of or both ofthe lower side and the upper side of a metal film of Al, Cu, or thelike. Alternatively, the conductive film for forming the sourceelectrode layer and the drain electrode layer may be formed using aconductive metal oxide. As the conductive metal oxide, indium oxide(In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), indium oxide-tin oxide(In₂O₃—SnO₂), indium oxide-zinc oxide (In₂O₃—ZnO), or any of these metaloxide materials in which silicon oxide is contained can be used.

Through a photolithography process, a resist mask is formed over theconductive film, and selective etching is performed thereon, so that thesource electrode layer 405 a and the drain electrode layer 405 b areformed, and then, the resist mask is removed (see FIG. 1D).

Next, the insulating layer 407 is formed over the gate electrode layer401, the sidewall insulating layers 412 a to 412 d, the source electrodelayer 405 a, and the drain electrode layer 405 b (see FIG. 1E). Theinsulating layer 407 has either a single-layer structure or astacked-layer structure, and includes an aluminum oxide film.

The thickness of the aluminum oxide film included in the insulatinglayer 407 is greater than or equal to 30 nm and less than or equal to500 nm, preferably greater than or equal to 50 nm and less than or equalto 200 nm. The insulating layer 407 can be formed by a method by whichimpurities such as water or hydrogen are prevented from entering theinsulating layer 407, such as a sputtering method as appropriate. Ifhydrogen is contained in the insulating layer 407, entry of hydrogeninto the oxide semiconductor film or extraction of oxygen from the oxidesemiconductor film by hydrogen is caused; thus, the resistance of theoxide semiconductor film might lower (the conductivity of the same mightbecome n-type) and a parasitic channel might be formed. Therefore, it isimportant that a film formation method in which hydrogen is not used isemployed in order to form the insulating layer 407 containing as littlehydrogen as possible.

The aluminum oxide film preferably has a region containing an excess ofoxygen as compared to the stoichiometric composition ratio of thealuminum oxide in a crystalline state. In that case, the oxygen contentis greater than that in the stoichiometric oxide aluminum oxide orhigher than that in the single crystal state. In some cases, oxygenexists between lattices of the aluminum oxide. Supposing that thecomposition is expressed by AlO_(x) (x>0), an aluminum oxide film havingan oxygen excess region where x exceeds 3/2 is preferably used. Such anoxygen excess region exists at least in part (including its interface)of the aluminum oxide film.

In this embodiment, an aluminum oxide film with a thickness of 100 nm isformed as the insulating layer 407 by a sputtering method. The formationof the aluminum oxide film by a sputtering method can be performed in arare gas (typically argon) atmosphere, an oxygen atmosphere, or a mixedatmosphere of a rare gas and oxygen.

To remove residual moisture from the deposition chamber for theinsulating layer 407 like in the formation of the oxide semiconductorfilm, an entrapment vacuum pump (such as a cryopump) is preferably used.This is because the deposition chamber evacuated using a cryopumpenables the impurity concentration of the insulating layer 407 to below. As an evacuation unit for removing moisture remaining in thedeposition chamber for the insulating layer 407, a turbo molecular pumpprovided with a cold trap may also be used.

A high-purity gas from which impurities such as hydrogen, water, ahydroxyl group, and hydride have been removed is preferably used as asputtering gas for the deposition of the insulating layer 407.

The insulating layer 407 can have a stacked-layer structure includingthe aluminum oxide film and an inorganic insulating film such as,typically, a silicon oxide film, a silicon oxynitride film, an aluminumoxynitride film, or a gallium oxide film. FIG. 7A illustrates an examplein which the insulating layer 407 in the transistor 440 has astacked-layer structure of insulating layers 407 a and 407 b.

As shown in FIG. 7A, the insulating layer 407 a is formed over the gateinsulating layer 401, the sidewall insulating layers 412 a and 412 b,the source electrode layer 405 a, and the drain electrode layer 405 b,and the insulating layer 407 b is formed over the insulating layer 407a. For example, in this embodiment, a silicon oxide film which has aregion containing an excess of oxygen as compared to the stoichiometriccomposition ratio of the silicon oxide in a crystalline state is used asthe insulating layer 407 a, and an aluminum oxide film is used as theinsulating layer 407 b.

Next, heat treatment is performed on the amorphous oxide semiconductorfilm 443 to crystallize at least part of the amorphous oxidesemiconductor film 443, so that the crystalline oxide semiconductor film403 which includes a crystal having a c-axis which is substantiallyperpendicular to a top surface of the crystalline oxide semiconductorfilm 403 is formed.

The aluminum oxide film provided as the insulating layer 407 over theamorphous oxide semiconductor film 443 has a high shielding effect(blocking effect) of blocking penetration of both oxygen and impuritiessuch as hydrogen and moisture.

Therefore, in and after the manufacturing process, the aluminum oxidefilm functions as a protective film for preventing entry of an impuritysuch as hydrogen or moisture, which causes a change in characteristics,into the oxide semiconductor film (the amorphous oxide semiconductorfilm 443, the crystalline oxide semiconductor film 403) and release ofoxygen, which is a main component material of the oxide semiconductor,from the oxide semiconductor film (the amorphous oxide semiconductorfilm 443, the crystalline oxide semiconductor film 403).

Since the heat treatment for crystallizing the amorphous oxidesemiconductor film 443 is performed in the state where the amorphousoxide semiconductor film 443 is covered with the aluminum oxide filmprovided as the insulating layer 407, oxygen can be prevented from beingreleased from the amorphous oxide semiconductor film 443 by the heattreatment. Thus, the resulting crystalline oxide semiconductor film 403can maintain the amount of oxygen contained in the amorphous oxidesemiconductor film 443, and therefore has a region where the amount ofoxygen is excess as compared to the stoichiometric oxide semiconductorin a crystalline state.

Therefore, the crystalline oxide semiconductor film 403 has high puritybecause impurities such as hydrogen and moisture do not enter thecrystalline oxide semiconductor film 403, and has the region where theamount of oxygen is excess as compared to the stoichiometric oxidesemiconductor in a crystalline state because oxygen is prevented frombeing released therefrom.

If oxygen is eliminated from the crystalline oxide semiconductor film403, an oxygen vacancy is formed therein. An oxide semiconductor with noexcess oxygen cannot repair such an oxygen vacancy with other oxygen. Incontrast, since the crystalline oxide semiconductor film 403 accordingto one embodiment of the present invention is a CAAC-OS film containingan excess of oxygen, the excess of oxygen (which is preferably excess ascompared to the stoichiometric composition ratio) contained in the filmcan act to repair an oxygen vacancy in the crystalline oxidesemiconductor film 403 immediately.

Accordingly, the crystalline oxide semiconductor film 403 enables avariation in the threshold voltage V_(th) of the transistor and a shiftof the threshold voltage (ΔV_(th)) due to an oxygen vacancy to bereduced.

The heat treatment for crystallizing at least part of the amorphousoxide semiconductor film 443 is performed at a temperature(s) higherthan or equal to 300° C. and lower than or equal to 700° C., preferablyhigher than or equal to 450° C. and lower than or equal to 650° C.,further preferably higher than or equal to 500° C., still furtherpreferably higher than or equal to 550° C.

For example, the substrate is introduced into an electric furnace whichis one of heat treatment apparatuses, and heat treatment is performed onthe oxide semiconductor film at 450° C. for 1 hour in an oxygenatmosphere.

The heat treatment apparatus is not limited to an electric furnace, andinstead, any device for heating an object by heat conduction or heatradiation from a heating element such as a resistance heating elementmay be used. For example, an RTA (rapid thermal anneal) apparatus suchas a GRTA (gas rapid thermal anneal) apparatus or an LRTA (lamp rapidthermal anneal) apparatus can be used. The LRTA apparatus is anapparatus for heating an object by radiation of light (electromagneticwave) emitted from a lamp such as a halogen lamp, a metal halide lamp, axenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or ahigh pressure mercury lamp. The GRTA apparatus is an apparatus for heattreatment using a high-temperature gas. As the high-temperature gas, aninert gas which does not react with an object by heat treatment, such asnitrogen or a rare gas like argon, is used.

For example, as the heat treatment, GRTA may be performed as follows:the substrate is put in an inert gas heated at a high temperature ofbetween 650° C. and 700° C., and is heated for several minutes, and isthen taken out of the inert gas.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air in which the water content is 20 ppm orlower, preferably 1 ppm or lower, further preferably 10 ppb or lower),or a rare gas (argon, helium, or the like). It is preferable that water,hydrogen, or the like is not contained in the atmosphere of nitrogen,oxygen, ultra-dry air, or a rare gas. The purity of nitrogen, oxygen, orthe rare gas which is introduced into the heat treatment apparatus ispreferably 6N (99.9999%) or higher, further preferably 7N (99.99999%) orhigher (that is, the impurity concentration is preferably 1 ppm orlower, further preferably 0.1 ppm or lower).

In the crystalline oxide semiconductor film 403 which is highly purifiedand whose oxygen vacancy is repaired, impurities such as hydrogen andwater are sufficiently removed; the hydrogen concentration in thecrystalline oxide semiconductor film 403 is less than or equal to5×10¹⁹/cm³, preferably less than or equal to 5×10¹⁸/cm³. The hydrogenconcentration in the crystalline oxide semiconductor film 403 ismeasured by secondary ion mass spectrometry (SIMS).

The number of carriers in the crystalline oxide semiconductor film 403is extremely small (close to zero), and the carrier concentration isless than 1×10¹⁴/cm³, preferably less than 1×10¹²/cm³, furtherpreferably less than 1×10¹¹/cm³.

Through the above process, the transistor 440 is formed (see FIG. 1F).The transistor 440 includes the highly purified crystalline oxidesemiconductor film containing an excess of oxygen that repairs an oxygenvacancy. Therefore, the transistor 440 has less change in electriccharacteristics and thus is electrically stable.

The current value in the off state (off-state current value) of thetransistor 440 using the highly purified crystalline oxide semiconductorfilm 403 containing an excess of oxygen that repairs an oxygen vacancyaccording to this embodiment is less than or equal to 100 zA permicrometer of channel width at room temperature (1 zA(zeptoampere)=1×10⁻²¹ A), preferably less than or equal to 10 zA/μm,further preferably less than or equal to 1 zA/μm, still furtherpreferably less than or equal to 100 yA/μm.

In this manner, a semiconductor device using an oxide semiconductor withstable electric characteristics can be provided. Accordingly, a highlyreliable semiconductor device can be provided.

Embodiment 2

In this embodiment, one embodiment of a semiconductor device and amethod for manufacturing the semiconductor device is described withreference to FIGS. 2A to 2F. The description of the above embodiment canbe applied to the same portion as or a portion having a function similarto that in the above embodiment, and the same step as or a step having afunction similar to that in the above embodiment, and descriptionthereof is not repeated. In addition, detailed description of the sameportions is omitted.

Described in this embodiment is an example in which heat treatment isperformed on an amorphous oxide semiconductor film to crystallize atleast part thereof, whereby a crystalline oxide semiconductor filmcontaining a crystal having a c-axis which is substantiallyperpendicular to a top surface of the crystalline oxide semiconductorfilm is formed in a method for manufacturing a semiconductor deviceaccording to one embodiment of the present invention.

FIGS. 2A to 2F illustrate an example of a method for manufacturing thetransistor 440.

First, the insulating layer 436 is formed over the substrate 400. Next,an amorphous oxide semiconductor film 441 is formed over the insulatinglayer 436 (see FIG. 2A). The amorphous oxide semiconductor film 441 canbe formed by a similar material and a similar method to those of thecrystalline oxide semiconductor film 444 described in Embodiment 1;however, the substrate temperature is a temperature at whichcrystallization does not occur in film formation (preferably lower thanor equal to 200° C.).

Further, heat treatment may be performed on the amorphous oxidesemiconductor film 441 in order to remove excess hydrogen (includingwater and a hydroxyl group) (to perform dehydration or dehydrogenation).The temperature of the heat treatment is a temperature(s) at which theamorphous oxide semiconductor film 441 is not crystallized, andtypically higher than or equal to 250° C. and lower than or equal to400° C., preferably lower than or equal to 300° C.

The heat treatment for dehydration or dehydrogenation is preferablyperformed before the amorphous oxide semiconductor film 441 is processedinto an island shape, whereby oxygen contained in the insulating layer436 can be prevented from being released by the heat treatment.

In the heat treatment, it is preferable that water, hydrogen, or thelike is not contained in nitrogen or a rare gas such as helium, neon, orargon; or the purity of nitrogen or the rare gas such as helium, neon,or argon which is introduced into the heat treatment apparatus ispreferably 6N (99.9999%) or higher, further preferably 7N (99.99999%) orhigher (that is, the impurity concentration is preferably 1 ppm orlower, further preferably 0.1 ppm or lower).

In addition, after the amorphous oxide semiconductor film 441 is heatedby the heat treatment, a high-purity oxygen gas, a high-purity N₂O gas,or ultra dry air (the moisture amount is less than or equal to 20 ppm(−55° C. by conversion into a dew point), preferably less than or equalto 1 ppm, further preferably less than or equal to 10 ppb, according toa dew point meter of a cavity ring down laser spectroscopy (CRDS)system) may be introduced into the same furnace. It is preferable thatwater, hydrogen, or the like is not contained in the oxygen gas or theN₂O gas; or the purity of the oxygen gas or the N₂O gas which isintroduced into the heat treatment apparatus is preferably 6N or higher,further preferably 7N or higher (i.e., the impurity concentration in theoxygen gas or the N₂O gas is preferably 1 ppm or lower, furtherpreferably 0.1 ppm or lower). The oxygen gas or the N₂O gas acts tosupply oxygen that is a main component of the amorphous oxidesemiconductor and that is reduced by the step for removing an impurityfor dehydration or dehydrogenation, thereby making the amorphous oxidesemiconductor film 441 a highly purified, electrically i-type(intrinsic) oxide semiconductor.

Next, heat treatment is performed on the amorphous oxide semiconductorfilm 441 to crystallize at least part of the amorphous oxidesemiconductor film 441, so that a crystalline oxide semiconductor filmincluding a crystal having a c-axis which is substantially perpendicularto a top surface of the crystalline oxide semiconductor film is formed.

The heat treatment for crystallizing at least part of the amorphousoxide semiconductor film 441 is performed at a temperature(s) higherthan or equal to 300° C. and lower than or equal to 700° C., preferablyhigher than or equal to 450° C. and lower than or equal to 650° C.,further preferably higher than or equal to 500° C., still furtherpreferably higher than or equal to 550° C.

For example, the substrate is introduced into an electric furnace whichis one of heat treatment apparatuses, and heat treatment is performed onthe amorphous oxide semiconductor film 441 at 650° C. for 1 hour in annitrogen atmosphere.

The heat treatment apparatus is not limited to an electric furnace, andinstead, any device for heating an object by heat conduction or heatradiation from a heating element such as a resistance heating elementmay be used. For example, an RTA (rapid thermal anneal) apparatus suchas a GRTA (gas rapid thermal anneal) apparatus or an LRTA (lamp rapidthermal anneal) apparatus can be used. The LRTA apparatus is anapparatus for heating an object by radiation of light (electromagneticwave) emitted from a lamp such as a halogen lamp, a metal halide lamp, axenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or ahigh pressure mercury lamp. The GRTA apparatus is an apparatus for heattreatment using a high-temperature gas. As the high-temperature gas, aninert gas which does not react with an object by heat treatment, such asnitrogen or a rare gas like argon, is used.

For example, as the heat treatment, GRTA may be performed as follows:the substrate is put in an inert gas heated at a high temperature ofbetween 650° C. and 700° C., and is heated for several minutes, and isthen taken out of the inert gas.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air in which the water content is 20 ppm orlower, preferably 1 ppm or lower, further preferably 10 ppb or lower),or a rare gas (argon, helium, or the like). It is preferable that water,hydrogen, or the like is not contained in the atmosphere of nitrogen,oxygen, ultra-dry air, or a rare gas. The purity of nitrogen, oxygen, orthe rare gas which is introduced into the heat treatment apparatus ispreferably 6N (99.9999%) or higher, further preferably 7N (99.99999%) orhigher (that is, the impurity concentration is preferably 1 ppm orlower, further preferably 0.1 ppm or lower).

The crystalline oxide semiconductor film is processed into an islandshape to form the crystalline oxide semiconductor film 413. Next, thegate insulating layer 442 is formed to cover the crystalline oxidesemiconductor film 413 (see FIG. 2B).

In this embodiment, such heat treatment for dehydration ordehydrogenation may be performed in the manufacturing process of thetransistor 440 anytime after formation of the amorphous oxidesemiconductor film 443 before addition of oxygen into the crystallineoxide semiconductor film 413.

The heat treatment for dehydration or dehydrogenation is preferablyperformed before the crystalline oxide semiconductor film is processedinto an island shape to be the crystalline oxide semiconductor film 413,whereby oxygen contained in the insulating layer 436 can be preventedfrom being released by the heat treatment.

Next, the oxygen 431 (including at least one of an oxygen radical, anoxygen atom, and an oxygen ion) is added into the crystalline oxidesemiconductor film 413; thus, oxygen is supplied to the crystallineoxide semiconductor film 413.

In this embodiment, the oxygen 431 is added into the crystalline oxidesemiconductor film 413 through the gate insulating layer 442 by an ionimplantation method. The addition of the oxygen 431 amorphizes at leastpart of the crystalline oxide semiconductor film 413, so that theamorphous oxide semiconductor film 443 having a region where the amountof oxygen is excess as compared to the stoichiometric composition ratioof the oxide semiconductor in a crystalline state is formed (see FIG.2C).

An oxygen vacancy in the amorphous oxide semiconductor film 443 can berepaired by the oxygen 431.

Next, the gate electrode layer 401 is formed over the gate insulatinglayer 442.

The sidewall insulating layers 412 a and 412 b are formed on the sidesurface of the gate electrode layer 401, and the gate insulating layer402 is formed.

The gate insulating layer 402 can be formed by etching the gateinsulating layer 442 with use of the gate electrode layer 401 and thesidewall insulating layers 412 a and 412 b as a mask.

Next, a conductive film for forming a source electrode layer and a drainelectrode layer (including a wiring formed of the same layer) is formedover part of the sidewall insulating layer 412 a, part of the sidewallinsulating layer 412 b, and the amorphous oxide semiconductor film 443.

Through a photolithography process, a resist mask is formed over theconductive film, and selective etching is performed thereon, so that thesource electrode layer 405 a and the drain electrode layer 405 b areformed, and then, the resist mask is removed (see FIG. 2D).

Next, the insulating layer 407 is formed over the gate electrode layer401, the sidewall insulating layers 412 a to 412 d, the source electrodelayer 405 a, and the drain electrode layer 405 b (see FIG. 2E). Theinsulating layer 407 has either a single-layer structure or astacked-layer structure, and includes an aluminum oxide film.

In this embodiment, a 100-nm-thick aluminum oxide film is formed as theinsulating layer 407 by a sputtering method.

Next, heat treatment is performed on the amorphous oxide semiconductorfilm 443 to crystallize at least part of the amorphous oxidesemiconductor film 443, so that the crystalline oxide semiconductor film403 which includes a crystal having a c-axis which is substantiallyperpendicular to a top surface of the crystalline oxide semiconductorfilm 403 is formed.

The aluminum oxide film provided as the insulating layer 407 over theamorphous oxide semiconductor film 443 has a high shielding effect(blocking effect) of blocking penetration of both oxygen and impuritiessuch as hydrogen and moisture.

Therefore, in and after the manufacturing process, the aluminum oxidefilm functions as a protective film for preventing entry of an impuritysuch as hydrogen or moisture, which causes a change in characteristics,into the oxide semiconductor film (the amorphous oxide semiconductorfilm 443, the crystalline oxide semiconductor film 403) and release ofoxygen, which is a main component material of the oxide semiconductor,from the oxide semiconductor film (the amorphous oxide semiconductorfilm 443, the crystalline oxide semiconductor film 403).

Since the heat treatment for crystallizing the amorphous oxidesemiconductor film 443 is performed in the state where the amorphousoxide semiconductor film 443 is covered with the aluminum oxide filmprovided as the insulating layer 407, oxygen can be prevented from beingreleased from the amorphous oxide semiconductor film 443 by the heattreatment. Thus, the resulting crystalline oxide semiconductor film 403can maintain the amount of oxygen contained in the amorphous oxidesemiconductor film 443, and therefore has a region where the amount ofoxygen is excess as compared to the stoichiometric oxide semiconductorin a crystalline state.

Therefore, the crystalline oxide semiconductor film 403 has high puritybecause impurities such as hydrogen and moisture do not enter thecrystalline oxide semiconductor film 403, and has the region where theamount of oxygen is excess as compared to the stoichiometric oxidesemiconductor in a crystalline state because oxygen is prevented frombeing released therefrom.

If oxygen is eliminated from the crystalline oxide semiconductor film403, an oxygen vacancy is formed therein. An oxide semiconductor with noexcess oxygen cannot repair such an oxygen vacancy with other oxygen. Incontrast, since the crystalline oxide semiconductor film 403 accordingto one embodiment of the present invention is a CAAC-OS film containingan excess of oxygen, the excess of oxygen (which is preferably excess ascompared to the stoichiometric composition ratio) contained in the filmcan act to repair an oxygen vacancy in the crystalline oxidesemiconductor film 403 immediately.

Accordingly, the crystalline oxide semiconductor film 403 enables avariation in the threshold voltage V_(th) of the transistor and a shiftof the threshold voltage (ΔV_(th)) due to an oxygen vacancy to bereduced.

Through the above process, the transistor 440 is formed (see FIG. 2F).The transistor 440 includes the highly purified crystalline oxidesemiconductor film containing an excess of oxygen that repairs an oxygenvacancy. Therefore, the transistor 440 has less change in electriccharacteristics and thus is electrically stable.

The current value in the off state (off-state current value) of thetransistor 440 using the highly purified crystalline oxide semiconductorfilm 403 containing an excess of oxygen that repairs an oxygen vacancyaccording to this embodiment is less than or equal to 100 zA permicrometer of channel width at room temperature (1 zA(zeptoampere)=1×10⁻²¹ A), preferably less than or equal to 10 zA/μm,further preferably less than or equal to 1 zA/μm, still furtherpreferably less than or equal to 100 yA/μm.

In this manner, a semiconductor device using an oxide semiconductor withstable electric characteristics can be provided. Accordingly, a highlyreliable semiconductor device can be provided.

Embodiment 3

In this embodiment, one embodiment of a semiconductor device and amethod for manufacturing the semiconductor device is described withreference to FIGS. 3A to 3E. The description of the above embodiment canbe applied to the same portion as or a portion having a function similarto that in the above embodiment, and the same step as or a step having afunction similar to that in the above embodiment, and descriptionthereof is not repeated. In addition, detailed description of the sameportions is omitted.

Described in this embodiment is an example in which addition of oxygeninto a crystalline oxide semiconductor film is performed through a gateinsulating layer after a gate electrode layer is formed in a method formanufacturing a semiconductor device according to one embodiment of thepresent invention.

FIGS. 3A to 3E illustrate an example of a method for manufacturing thetransistor 440.

First, the insulating layer 436 is formed over the substrate 400. Next,the crystalline oxide semiconductor film 413 is formed over theinsulating layer 436. The crystalline oxide semiconductor film 413 canbe formed by a similar material and a similar method to those of thecrystalline oxide semiconductor film 413 described in Embodiment 1 orEmbodiment 2. Then, the gate insulating layer 442 is formed to cover thecrystalline oxide semiconductor film 413.

Next, the gate electrode layer 401 is formed over the gate insulatinglayer 442 (see FIG. 3A).

Further, heat treatment may be performed on the crystalline oxidesemiconductor film 413 in order to remove excess hydrogen (includingwater and a hydroxyl group) (to perform dehydration or dehydrogenation).

Next, the oxygen 431 (including at least one of an oxygen radical, anoxygen atom, and an oxygen ion) is added into the crystalline oxidesemiconductor film 413; thus oxygen is supplied to the crystalline oxidesemiconductor film 413.

In this embodiment, the oxygen 431 is added into the crystalline oxidesemiconductor film 413 through the gate insulating layer 442 by an ionimplantation method after the gate electrode layer 401 is formed. Theaddition of the oxygen 431 amorphizes at least part of the crystallineoxide semiconductor film 413, so that the amorphous oxide semiconductorfilm 443 having a region where the amount of oxygen is excess ascompared to the stoichiometric composition ratio of the oxidesemiconductor in a crystalline state is formed (see FIG. 3B).

Although there is a case where the oxygen 431 is not added in a regionof the crystalline oxide semiconductor film 413 which is overlapped withthe gate electrode layer 401 due to the existence of the gate electrodelayer 401, oxygen added into the amorphous oxide semiconductor film 443can be diffused also to the region of the crystalline oxidesemiconductor film 413 which is overlapped with the gate electrode layer401 by heat treatment for crystallizing the amorphous oxidesemiconductor film 443 because the width of the gate electrode layer 401is small (for example, in the submicron order).

An oxygen vacancy in the amorphous oxide semiconductor film 443 can berepaired by the oxygen 431.

The sidewall insulating layers 412 a and 412 b are formed on the sidesurface of the gate electrode layer 401, and the gate insulating layer402 is formed.

The gate insulating layer 402 can be formed by etching the gateinsulating layer 442 with use of the gate electrode layer 401 and thesidewall insulating layers 412 a and 412 b as a mask.

Next, a conductive film for forming a source electrode layer and a drainelectrode layer (including a wiring formed of the same layer) is formedover part of the sidewall insulating layer 412 a, part of the sidewallinsulating layer 412 b, and the amorphous oxide semiconductor film 443.

Through a photolithography process, a resist mask is formed over theconductive film, and selective etching is performed thereon, so that thesource electrode layer 405 a and the drain electrode layer 405 b areformed, and then, the resist mask is removed (see FIG. 3C).

Next, the insulating layer 407 is formed over the gate electrode layer401, the sidewall insulating layers 412 a to 412 d, the source electrodelayer 405 a, and the drain electrode layer 405 b (see FIG. 3D). Theinsulating layer 407 has either a single-layer structure or astacked-layer structure, and includes an aluminum oxide film.

In this embodiment, a 100-nm-thick aluminum oxide film is formed as theinsulating layer 407 by a sputtering method.

Next, heat treatment is performed on the amorphous oxide semiconductorfilm 443 to crystallize at least part of the amorphous oxidesemiconductor film 443, so that the crystalline oxide semiconductor film403 which includes a crystal having a c-axis which is substantiallyperpendicular to a top surface of the crystalline oxide semiconductorfilm 403 is formed. Also with this heat treatment, oxygen is diffusedthroughout the amorphous oxide semiconductor film 443, so that oxygen issupplied throughout the film.

The aluminum oxide film provided as the insulating layer 407 over theamorphous oxide semiconductor film 443 has a high shielding effect(blocking effect) of blocking penetration of both oxygen and impuritiessuch as hydrogen and moisture.

Therefore, in and after the manufacturing process, the aluminum oxidefilm functions as a protective film for preventing entry of an impuritysuch as hydrogen or moisture, which causes a change in characteristics,into the oxide semiconductor film (the amorphous oxide semiconductorfilm 443, the crystalline oxide semiconductor film 403) and release ofoxygen, which is a main component material of the oxide semiconductor,from the oxide semiconductor film (the amorphous oxide semiconductorfilm 443, the crystalline oxide semiconductor film 403).

Since the heat treatment for crystallizing the amorphous oxidesemiconductor film 443 is performed in the state where the amorphousoxide semiconductor film 443 is covered with the aluminum oxide filmprovided as the insulating layer 407, oxygen can be prevented from beingreleased from the amorphous oxide semiconductor film 443 by the heattreatment. Thus, the resulting crystalline oxide semiconductor film 403can maintain the amount of oxygen contained in the amorphous oxidesemiconductor film 443, and therefore has a region where the amount ofoxygen is excess as compared to the stoichiometric oxide semiconductorin a crystalline state.

Therefore, the crystalline oxide semiconductor film 403 has high puritybecause impurities such as hydrogen and moisture do not enter thecrystalline oxide semiconductor film 403, and has the region where theamount of oxygen is excess as compared to the stoichiometric oxidesemiconductor in a crystalline state because oxygen is prevented frombeing released therefrom.

If oxygen is eliminated from the crystalline oxide semiconductor film403, an oxygen vacancy is formed therein. An oxide semiconductor with noexcess oxygen cannot repair such an oxygen vacancy with other oxygen. Incontrast, since the crystalline oxide semiconductor film 403 accordingto one embodiment of the present invention is a CAAC-OS film containingan excess of oxygen, the excess of oxygen (which is preferably excess ascompared to the stoichiometric composition ratio) contained in the filmcan act to repair an oxygen vacancy in the crystalline oxidesemiconductor film 403 immediately.

Accordingly, the crystalline oxide semiconductor film 403 enables avariation in the threshold voltage V_(th) of the transistor 440 and ashift of the threshold voltage (ΔV_(th)) due to an oxygen vacancy to bereduced.

Through the above process, the transistor 440 is formed (see FIG. 3E).The transistor 440 includes the highly purified crystalline oxidesemiconductor film containing an excess of oxygen that repairs an oxygenvacancy. Therefore, the transistor 440 has less change in electriccharacteristics and thus is electrically stable.

The current value in the off state (off-state current value) of thetransistor 440 using the highly purified crystalline oxide semiconductorfilm 403 containing an excess of oxygen that repairs an oxygen vacancyaccording to this embodiment is less than or equal to 100 zA permicrometer of channel width at room temperature (1 zA(zeptoampere)=1×10⁻²¹ A), preferably less than or equal to 10 zA/μm,further preferably less than or equal to 1 zA/μm, still furtherpreferably less than or equal to 100 yA/μm.

In this manner, a semiconductor device using an oxide semiconductor withstable electric characteristics can be provided. Accordingly, a highlyreliable semiconductor device can be provided.

Embodiment 4

In this embodiment, one embodiment of a semiconductor device and amethod for manufacturing the semiconductor device is described withreference to FIGS. 4A to 4E. The description of the above embodiment canbe applied to the same portion as or a portion having a function similarto that in the above embodiment, and the same step as or a step having afunction similar to that in the above embodiment, and descriptionthereof is not repeated. In addition, detailed description of the sameportions is omitted.

Described in this embodiment is an example in which addition of oxygeninto a crystalline oxide semiconductor film is performed through aninsulating layer provided over a transistor in a method formanufacturing a semiconductor device according to one embodiment of thepresent invention.

FIGS. 4A to 4E illustrate an example of a method for manufacturing atransistor 410.

First, the insulating layer 436 is formed over the substrate 400. Next,the crystalline oxide semiconductor film 413 is formed over theinsulating layer 436. The crystalline oxide semiconductor film 413 canbe formed by a similar material and a similar method to those of thecrystalline oxide semiconductor film 413 described in Embodiment 1 orEmbodiment 2. Then, the gate insulating layer 442 is formed to cover thecrystalline oxide semiconductor film 413.

Next, the gate electrode layer 401 is formed over the gate insulatinglayer 442 (see FIG. 4A).

In this embodiment, an example in which a sidewall insulating layer isnot formed and the gate insulating layer 442 is not processed into anisland shape and is used as a continuous film is described.

Further, heat treatment may be performed on the crystalline oxidesemiconductor film 413 in order to remove excess hydrogen (includingwater and a hydroxyl group) (to perform dehydration or dehydrogenation).

Next, the insulating layer 407 is formed over the gate insulating layer442 and the gate electrode layer 401 (see FIG. 4B). The insulating layer407 has either a single-layer structure or a stacked-layer structure,and includes an aluminum oxide film.

In this embodiment, a 100-nm-thick aluminum oxide film is formed as theinsulating layer 407 by a sputtering method.

Next, the oxygen 431 (including at least one of an oxygen radical, anoxygen atom, and an oxygen ion) is added into the crystalline oxidesemiconductor film 413; thus, oxygen is supplied to the crystallineoxide semiconductor film 413.

In this embodiment, the oxygen 431 is added into the crystalline oxidesemiconductor film 413 through the gate insulating layer 442 and theinsulating layer 407 by an ion implantation method after the insulatinglayer 407 is formed. The addition of the oxygen 431 amorphizes at leastpart of the crystalline oxide semiconductor film 413, so that theamorphous oxide semiconductor film 443 having a region where the amountof oxygen is excess as compared to the stoichiometric composition ratioof the oxide semiconductor in a crystalline state is formed (see FIG.4C).

Although there is a case where the oxygen 431 is not added in a regionof the crystalline oxide semiconductor film 413 which is overlapped withthe gate electrode layer 401 due to the existence of the gate electrodelayer 401, oxygen added into the amorphous oxide semiconductor film 443can be diffused also to the region of the amorphous oxide semiconductorfilm 443 which is overlapped with the gate electrode layer 401 by heattreatment for crystallizing the amorphous oxide semiconductor film 443because the width of the gate electrode layer 401 is small (for example,0.35 μm).

An oxygen vacancy in the amorphous oxide semiconductor film 443 can berepaired by the oxygen 431.

Next, heat treatment is performed on the amorphous oxide semiconductorfilm 443 to crystallize at least part of the amorphous oxidesemiconductor film 443, so that the crystalline oxide semiconductor film403 which includes a crystal having a c-axis which is substantiallyperpendicular to a top surface of the crystalline oxide semiconductorfilm 403 is formed (see FIG. 4D). Also with this heat treatment, oxygenis diffused throughout the amorphous oxide semiconductor film 443, sothat oxygen is supplied throughout the film.

The aluminum oxide film provided as the insulating layer 407 over theamorphous oxide semiconductor film 443 has a high shielding effect(blocking effect) of blocking penetration of both oxygen and impuritiessuch as hydrogen and moisture.

Therefore, in and after the manufacturing process, the aluminum oxidefilm functions as a protective film for preventing entry of an impuritysuch as hydrogen or moisture, which causes a change in characteristics,into the oxide semiconductor film (the amorphous oxide semiconductorfilm 443, the crystalline oxide semiconductor film 403) and release ofoxygen, which is a main component material of the oxide semiconductor,from the oxide semiconductor film (the amorphous oxide semiconductorfilm 443, the crystalline oxide semiconductor film 403).

Since the heat treatment for crystallizing the amorphous oxidesemiconductor film 443 is performed in the state where the amorphousoxide semiconductor film 443 is covered with the aluminum oxide filmprovided as the insulating layer 407, oxygen can be prevented from beingreleased from the amorphous oxide semiconductor film 443 by the heattreatment. Thus, the resulting crystalline oxide semiconductor film 403can maintain the amount of oxygen contained in the amorphous oxidesemiconductor film 443, and therefore has a region where the amount ofoxygen is excess as compared to the stoichiometric oxide semiconductorin a crystalline state.

Therefore, the crystalline oxide semiconductor film 403 has high puritybecause impurities such as hydrogen and moisture do not enter thecrystalline oxide semiconductor film 403, and has the region where theamount of oxygen is excess as compared to the stoichiometric oxidesemiconductor in a crystalline state because oxygen is prevented frombeing released therefrom.

If oxygen is eliminated from the crystalline oxide semiconductor film403, an oxygen vacancy is formed therein. An oxide semiconductor with noexcess oxygen cannot repair such an oxygen vacancy with other oxygen. Incontrast, since the crystalline oxide semiconductor film 403 accordingto one embodiment of the present invention is a CAAC-OS film containingan excess of oxygen, the excess of oxygen (which is preferably excess ascompared to the stoichiometric composition ratio) contained in the filmcan act to repair an oxygen vacancy in the crystalline oxidesemiconductor film 403 immediately.

Accordingly, the crystalline oxide semiconductor film 403 enables avariation in the threshold voltage V_(th) of the transistor 410 and ashift of the threshold voltage (ΔV_(th)) due to an oxygen vacancy to bereduced.

Further, a planarization insulating film may be formed thereover inorder to reduce surface roughness due to the transistor. For theplanarization insulating film, an organic material such as polyimide,acrylic, or benzocyclobutene can be used. Other than such an organicmaterial, it is also possible to use a low-dielectric constant material(a low-k material) or the like. The planarization insulating film may beformed by stacking a plurality of insulating films formed using thesematerials.

In this embodiment, a planarization insulating film 415 is formed overthe insulating layer 407. Further, openings reaching the crystallineoxide semiconductor film 403 are formed in the gate insulating layer442, the insulating layer 407, and the planarization insulating film415, and the source electrode layer 405 a and the drain electrode layer405 b are formed so as to be electrically connected to the crystallineoxide semiconductor film 403 through the openings.

Through the above process, the transistor 410 is formed (see FIG. 4E).The transistor 410 includes the highly purified crystalline oxidesemiconductor film containing an excess of oxygen that repairs an oxygenvacancy. Therefore, the transistor 410 has less change in electriccharacteristics and thus is electrically stable.

The insulating layer 407 can have a stacked-layer structure includingthe aluminum oxide film and an inorganic insulating film such as,typically, a silicon oxide film, a silicon oxynitride film, an aluminumoxynitride film, or a gallium oxide film. FIG. 7B illustrates an examplein which the insulating layer 407 in the transistor 410 has astacked-layer structure of the insulating layers 407 a and 407 b.

As shown in FIG. 7B, the insulating layer 407 a is formed over the gateinsulating layer 442 and the gate electrode layer 401, and theinsulating layer 407 b is formed over the insulating layer 407 a. Forexample, in this embodiment, a silicon oxide film which has a regioncontaining an excess of oxygen as compared to the stoichiometriccomposition ratio of the silicon oxide in a crystalline state is used asthe insulating layer 407 a, and an aluminum oxide film is used as theinsulating layer 407 b.

In the case the insulating layer 407 has the stacked-layer structure ofthe insulating layers 407 a and 407 b, the addition of oxygen into thecrystalline oxide semiconductor film 413 can be performed through thestacked insulating layers 407 a and 407 b.

The current value in the off state (off-state current value) of thetransistor 410 using the highly purified crystalline oxide semiconductorfilm 403 containing an excess of oxygen that repairs an oxygen vacancyaccording to this embodiment is less than or equal to 100 zA permicrometer of channel width at room temperature (1 zA(zeptoampere)=1×10⁻²¹ A), preferably less than or equal to 10 zA/μm,further preferably less than or equal to 1 zA/μm, still furtherpreferably less than or equal to 100 yA/μm.

In this manner, a semiconductor device using an oxide semiconductor withstable electric characteristics can be provided. Accordingly, a highlyreliable semiconductor device can be provided.

Embodiment 5

In this embodiment, one embodiment of a semiconductor device and amethod for manufacturing the semiconductor device is described withreference to FIGS. 5A to 5F. The description of the above embodiment canbe applied to the same portion as or a portion having a function similarto that in the above embodiment, and the same step as or a step having afunction similar to that in the above embodiment, and descriptionthereof is not repeated. In addition, detailed description of the sameportions is omitted.

Described in this embodiment is an example in which the structure ofconnection between a source and drain electrode layers and a crystallineoxide semiconductor film is different from Embodiment 1.

FIGS. 5A to 5F illustrate an example of a method for manufacturing thetransistor 430.

First, the insulating layer 436 is formed over the substrate 400.

Next, a conductive film for forming a source electrode layer and a drainelectrode layer (including a wiring formed of the same layer) is formedover the insulating layer 436.

Through a photolithography process, a resist mask is formed over theconductive film, and selective etching is performed thereon, so that thesource electrode layer 405 a and the drain electrode layer 405 b areformed, and then, the resist mask is removed (see FIG. 5A).

Then, the crystalline oxide semiconductor film 413 is formed over theinsulating layer 436, the source electrode layer 405 a, and the drainelectrode layer 405 b (see FIG. 5B). The crystalline oxide semiconductorfilm 413 can be formed by a similar material and a similar method tothose of the crystalline oxide semiconductor film 413 described inEmbodiment 1 or Embodiment 2. Then, the gate insulating layer 402 isformed to cover the crystalline oxide semiconductor film 413 (see FIG.5C).

Further, heat treatment may be performed on the crystalline oxidesemiconductor film 413 in order to remove excess hydrogen (includingwater and a hydroxyl group) (to perform dehydration or dehydrogenation).

Next, the oxygen 431 (including at least one of an oxygen radical, anoxygen atom, and an oxygen ion) is added into the crystalline oxidesemiconductor film 413; thus, oxygen is supplied to the crystallineoxide semiconductor film 413.

In this embodiment, the oxygen 431 is added into the crystalline oxidesemiconductor film 413 through the gate insulating layer 402 by an ionimplantation method. The addition of the oxygen 431 amorphizes at leastpart of the crystalline oxide semiconductor film 413, so that theamorphous oxide semiconductor film 443 having a region where the amountof oxygen is excess as compared to the stoichiometric composition ratioof the oxide semiconductor in a crystalline state is formed (see FIG.5D).

An oxygen vacancy in the amorphous oxide semiconductor film 443 can berepaired by the oxygen 431.

Next, the gate electrode layer 401 is formed over the gate insulatinglayer 402.

In the example described in this embodiment, a sidewall insulating layeris not provided on the side of the gate electrode layer 401; however, asidewall insulating layer may be provided and the gate insulating layer402 may be processed into an island shape as described in Embodiment 1.

Next, the insulating layer 407 is formed over the gate insulating layer402 and the gate electrode layer 401 (see FIG. 5E). The insulating layer407 has either a single-layer structure or a stacked-layer structure,and includes an aluminum oxide film.

In this embodiment, a 100-nm-thick aluminum oxide film is formed as theinsulating layer 407 by a sputtering method.

Next, heat treatment is performed on the amorphous oxide semiconductorfilm 443 to crystallize at least part of the amorphous oxidesemiconductor film 443, so that the crystalline oxide semiconductor film403 which includes a crystal having a c-axis which is substantiallyperpendicular to a top surface of the crystalline oxide semiconductorfilm 403 is formed.

The aluminum oxide film provided as the insulating layer 407 over theamorphous oxide semiconductor film 443 has a high shielding effect(blocking effect) of blocking penetration of both oxygen and impuritiessuch as hydrogen and moisture.

Therefore, in and after the manufacturing process, the aluminum oxidefilm functions as a protective film for preventing entry of an impuritysuch as hydrogen or moisture, which causes a change in characteristics,into the oxide semiconductor film (the amorphous oxide semiconductorfilm 443, the crystalline oxide semiconductor film 403) and release ofoxygen, which is a main component material of the oxide semiconductor,from the oxide semiconductor film (the amorphous oxide semiconductorfilm 443, the crystalline oxide semiconductor film 403).

Since the heat treatment for crystallizing the amorphous oxidesemiconductor film 443 is performed in the state where the amorphousoxide semiconductor film 443 is covered with the aluminum oxide filmprovided as the insulating layer 407, oxygen can be prevented from beingreleased from the amorphous oxide semiconductor film 443 by the heattreatment. Thus, the resulting crystalline oxide semiconductor film 403can maintain the amount of oxygen contained in the amorphous oxidesemiconductor film 443, and therefore has a region where the amount ofoxygen is excess as compared to the stoichiometric oxide semiconductorin a crystalline state.

Therefore, the crystalline oxide semiconductor film 403 has high puritybecause impurities such as hydrogen and moisture do not enter thecrystalline oxide semiconductor film 403, and has the region where theamount of oxygen is excess as compared to the stoichiometric oxidesemiconductor in a crystalline state because oxygen is prevented frombeing released therefrom.

If oxygen is eliminated from the crystalline oxide semiconductor film403, an oxygen vacancy is formed therein. An oxide semiconductor with noexcess oxygen cannot repair such an oxygen vacancy with other oxygen. Incontrast, since the crystalline oxide semiconductor film 403 accordingto one embodiment of the present invention is a CAAC-OS film containingan excess of oxygen, the excess of oxygen (which is preferably excess ascompared to the stoichiometric composition ratio) contained in the filmcan act to repair an oxygen vacancy in the crystalline oxidesemiconductor film 403 immediately.

Accordingly, the crystalline oxide semiconductor film 403 enables avariation in the threshold voltage V_(th) of the transistor 430 and ashift of the threshold voltage (ΔV_(th)) due to an oxygen vacancy to bereduced.

The insulating layer 407 can have a stacked-layer structure includingthe aluminum oxide film and an inorganic insulating film such as,typically, a silicon oxide film, a silicon oxynitride film, an aluminumoxynitride film, or a gallium oxide film. FIG. 7C illustrates an examplein which the insulating layer 407 in the transistor 430 has astacked-layer structure of the insulating layers 407 a and 407 b.

As shown in FIG. 7C, the insulating layer 407 a is formed over the gateinsulating layer 402 and the gate electrode layer 401, and theinsulating layer 407 b is formed over the insulating layer 407 a. Forexample, in this embodiment, a silicon oxide film which has a regioncontaining an excess of oxygen as compared to the stoichiometriccomposition ratio of the silicon oxide in a crystalline state is used asthe insulating layer 407 a, and an aluminum oxide film is used as theinsulating layer 407 b.

Through the above process, the transistor 430 is formed (see FIG. 5F).The transistor 430 includes the highly purified crystalline oxidesemiconductor film containing an excess of oxygen that repairs an oxygenvacancy. Therefore, the transistor 430 has less change in electriccharacteristics and thus is electrically stable.

The current value in the off state (off-state current value) of thetransistor 430 using the highly purified crystalline oxide semiconductorfilm 403 containing an excess of oxygen that repairs an oxygen vacancyaccording to this embodiment is less than or equal to 100 zA permicrometer of channel width at room temperature (1 zA(zeptoampere)=1×10⁻²¹ A), preferably less than or equal to 10 zA/μm,further preferably less than or equal to 1 zA/μm, still furtherpreferably less than or equal to 100 yA/μm.

In this manner, a semiconductor device using an oxide semiconductor withstable electric characteristics can be provided. Accordingly, a highlyreliable semiconductor device can be provided.

Embodiment 6

In this embodiment, one embodiment of a method for manufacturing asemiconductor device is described. The description of the aboveembodiment can be applied to the same portion as or a portion having afunction similar to that in the above embodiment, and the same step asor a step having a function similar to that in the above embodiment, anddescription thereof is not repeated. In addition, detailed descriptionof the same portions is omitted.

Described in this embodiment are examples of a step for adding oxygen,which is applicable in the manufacturing process of the transistor 430described in Embodiment 5.

FIG. 6A illustrates an example of adding the oxygen 431 directly intothe crystalline oxide semiconductor film 413 after the step shown inFIG. 5B. The addition of the oxygen 431 amorphizes at least part of thecrystalline oxide semiconductor film 413, so that the amorphous oxidesemiconductor film 443 having a region where the amount of oxygen isexcess as compared to the stoichiometric composition ratio of the oxidesemiconductor in a crystalline state is formed. An oxygen vacancy in theamorphous oxide semiconductor film 443 can be repaired by the oxygen431. In the case where oxygen is added directly into the crystallineoxide semiconductor film 413 which is bare as shown in FIG. 6A, plasmatreatment can be used.

FIG. 6B illustrates an example of adding the oxygen 431 into thecrystalline oxide semiconductor film 413 through the gate insulatinglayer 402 after the gate electrode layer 401 is formed over the gateinsulating layer 402. The addition of the oxygen 431 amorphizes at leastpart of the crystalline oxide semiconductor film 413, so that theamorphous oxide semiconductor film 443 having a region where the amountof oxygen is excess as compared to the stoichiometric composition ratioof the oxide semiconductor in a crystalline state is formed. An oxygenvacancy in the amorphous oxide semiconductor film 443 can be repaired bythe oxygen 431.

FIG. 6C illustrates an example of adding the oxygen 431 into thecrystalline oxide semiconductor film 413 through the gate insulatinglayer 402 and the insulating layer 407 after the insulating layer 407 isformed over the gate insulating layer 402 and the gate electrode layer401. The addition of the oxygen 431 amorphizes at least part of thecrystalline oxide semiconductor film 413, so that the amorphous oxidesemiconductor film 443 having a region where the amount of oxygen isexcess as compared to the stoichiometric composition ratio of the oxidesemiconductor in a crystalline state is formed. An oxygen vacancy in theamorphous oxide semiconductor film 443 can be repaired by the oxygen431.

As described above, the addition of oxygen into the crystalline oxidesemiconductor film can be performed anytime after dehydration ordehydrogenation is performed thereon. Further, the number of times ofthe addition of oxygen into the dehydrated or dehydrogenated oxidesemiconductor film is not limited.

The transistor manufactured through the above process includes thehighly purified crystalline oxide semiconductor film containing anexcess of oxygen that repairs an oxygen vacancy. Therefore, thetransistor has less change in electric characteristics and thus iselectrically stable.

A semiconductor device using an oxide semiconductor with stable electriccharacteristics can be provided. Accordingly, a highly reliablesemiconductor device can be provided.

This embodiment can be implemented combining with another embodiment asappropriate.

Embodiment 7

In this embodiment, one embodiment of a semiconductor device and amethod for manufacturing the semiconductor device is described withreference to FIGS. 8A to 8C. The description of the above embodiment canbe applied to the same portion as or a portion having a function similarto that in the above embodiment, and the same step as or a step having afunction similar to that in the above embodiment, and descriptionthereof is not repeated. In addition, detailed description of the sameportions is omitted.

Described in this embodiment are examples in which impurity regionsfunctioning as a source region and a drain region are formed in acrystalline oxide semiconductor film in a manufacturing method of asemiconductor device according to one embodiment of the presentinvention. The impurity regions functioning as a source region and adrain region can be formed by adding an impurity (also called a dopant)for changing the electrical conductivity into the crystalline oxidesemiconductor film.

The dopant concentration in each impurity region of the source regionand the drain region is preferably greater than or equal to 5×10¹⁸/cm³and less than or equal to 1×10²²/cm³.

The dopant is a Group 15 element and/or boron, specifically, one or moreselected from phosphorus, arsenic, antimony, and boron. As the method ofadding the dopant into the crystalline oxide semiconductor film, an iondoping method or an ion implantation method can be used.

The substrate may be heated while the dopant is added by an ion dopingmethod or an ion implantation method.

The addition of dopant into the crystalline oxide semiconductor film maybe performed plural times, and the number of kinds of dopant may beplural.

The addition of dopant may amorphizes the impurity region. In that case,the crystallinity can be recovered by performing heat treatment thereonafter the addition of dopant.

FIG. 8A illustrates a transistor 440 b in which impurity regions 404 aand 404 b functioning as a source region and a drain region are providedin the crystalline oxide semiconductor film 403 in the transistor 440described in Embodiments 1 to 3. The impurity regions 404 a and 404 bcan be formed by adding a dopant into the crystalline oxidesemiconductor film 403 with the gate electrode layer 401 and thesidewall insulating layers 412 a and 412 b as a mask before formation ofthe source electrode layer 405 a and the drain electrode layer 405 b.

FIG. 8B illustrates a transistor 410 b in which the impurity regions 404a and 404 b functioning as a source region and a drain region areprovided in the crystalline oxide semiconductor film 403 in thetransistor 410 described in Embodiment 4. The impurity regions 404 a and404 b can be formed by adding a dopant into the crystalline oxidesemiconductor film 403 with the gate electrode layer 401 as a mask.

FIG. 8C illustrates a transistor 430 b in which the impurity regions 404a and 404 b functioning as a source region and a drain region areprovided in the crystalline oxide semiconductor film 403 in thetransistor 430 described in Embodiment 5. The impurity regions 404 a and404 b can be formed by adding a dopant into the crystalline oxidesemiconductor film 403 with the gate electrode layer 401 as a mask.

With the impurity regions functioning as a source region and a drainregion, the electric field applied to a channel formation region betweenthe impurity regions can be attenuated. Further, by electricallyconnecting the electrode layer to the crystalline oxide semiconductorfilm in the impurity region, contact resistance between the electrodelayer and the crystalline oxide semiconductor film can be reduced.Consequently, the electrical characteristics of the transistor can beenhanced.

This embodiment can be implemented combining with another embodiment asappropriate.

Embodiment 8

A semiconductor device (also referred to as a display device) with adisplay function can be manufactured using the transistor an example ofwhich is described in any of Embodiments 1 to 7. Further, part or all ofthe driver circuitry which includes the transistor can be formed over asubstrate where the pixel portion is formed, whereby a system-on-panelcan be obtained.

In FIG. 20A, a sealant 4005 is provided so as to surround a pixelportion 4002 provided over a first substrate 4001, and the pixel portion4002 is sealed with a second substrate 4006. In FIG. 20A, a scan linedriver circuit 4004 and a signal line driver circuit 4003 each areformed using a single crystal semiconductor film or a polycrystallinesemiconductor film over another substrate, and mounted in a regiondifferent from the region surrounded by the sealant 4005 over the firstsubstrate 4001. Various signals and potentials are supplied to thesignal line driver circuit 4003, the scan line driver circuit 4004, andthe pixel portion 4002 through flexible printed circuits (FPCs) 4018 aand 4018 b.

In FIGS. 20B and 20C, the sealant 4005 is provided so as to surround thepixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001. The second substrate 4006 isprovided over the pixel portion 4002 and the scan line driver circuit4004. Hence, the pixel portion 4002 and the scan line driver circuit4004 are sealed together with the display element, by the firstsubstrate 4001, the sealant 4005, and the second substrate 4006.Further, in FIGS. 20B and 20C, the signal line driver circuit 4003 whichis formed using a single crystal semiconductor film or a polycrystallinesemiconductor film over another substrate is mounted in a region that isdifferent from the region surrounded by the sealant 4005 over the firstsubstrate 4001. In FIGS. 20B and 20C, various signals and potentials aresupplied to the signal line driver circuit 4003, the scan line drivercircuit 4004, and the pixel portion 4002 through an FPC 4018.

Although FIGS. 20B and 20C each illustrate an example in which thesignal line driver circuit 4003 is formed separately and mounted on thefirst substrate 4001, an embodiment of the present invention is notlimited to this structure. The scan line driver circuit may beseparately formed and then mounted, or only part of the signal linedriver circuit or only part of the scan line driver circuit may beseparately formed and then mounted.

The connection method of a separately formed driver circuit is notparticularly limited; a chip on glass (COG) method, a wire bondingmethod, a tape automated bonding (TAB) method, or the like can be used.FIG. 20A illustrates an example in which the signal line driver circuit4003 and the scan line driver circuit 4004 are mounted by a COG method;FIG. 20B illustrates an example in which the signal line driver circuit4003 is mounted by a COG method; FIG. 20C illustrates an example inwhich the signal line driver circuit 4003 is mounted by a TAB method.

The display device encompasses a panel in which a liquid crystal displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel.

The display device in this specification refers to an image displaydevice, a display device, or a light source (including a lightingdevice). Further, the display device also includes the following modulesin its category: a module to which a connector such as an FPC, a TABtape, or a TCP is attached; a module having a TAB tape or a TCP at thetip of which a printed wiring board is provided; and a module in whichan integrated circuit (IC) is directly mounted on a display element by aCOG method.

The pixel portion and the scan line driver circuit provided over thefirst substrate include a plurality of transistors to which any of thetransistors which are described in Embodiments 1 to 7 can be applied.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. The light-emitting element includes in itscategory an element whose luminance is controlled by a current or avoltage, and specifically encompasses an inorganic electroluminescent(EL) element, an organic EL element, and the like. Besides those,display medium whose contrast is changed by an electric effect, such aselectronic ink, can also be used.

An embodiment of a semiconductor device is described with reference toFIG. 20A and FIGS. 21A and 21B. FIGS. 21A and 21B correspond tocross-sectional views taken along line M-N in FIG. 20A.

As shown in FIGS. 21A and 21B, the semiconductor device has a connectionterminal electrode 4015 and a terminal electrode 4016, and theconnection terminal electrode 4015 and the terminal electrode 4016 areelectrically connected to a terminal included in the FPC 4018 through ananisotropic conductive film 4019.

The connection terminal electrode 4015 is formed of the same conductivefilm as a first electrode layer 4030, and the terminal electrode 4016 isformed of the same conductive film as a source and drain electrodelayers of transistors 4010 and 4011.

The pixel portion 4002 and the scan line driver circuit 4004 providedover the first substrate 4001 include a plurality of transistors. InFIGS. 21A and 21B, the transistor 4010 included in the pixel portion4002 and the transistor 4011 included in the scan line driver circuit4004 are shown as an example. An insulating layer 4020 is provided overthe transistors 4010 and 4011 in FIG. 21A, and an insulating layer 4021is further provided in FIG. 21B. An insulating film 4023 is aninsulating film serving as a base film.

In this embodiment, the transistor described in any of Embodiments 1 to7 can be applied to the transistor 4010, 4011.

Each of the transistors 4010 and 4011 includes a highly purifiedcrystalline oxide semiconductor film containing excess oxygen thatrepairs an oxygen vacancy. Therefore, change in the electriccharacteristics of the transistor 4010 and the transistor 4011 issuppressed, and the transistor 4010 and the transistor 4011 areelectrically stable.

Accordingly, highly reliable semiconductor devices can be provided asthe semiconductor device of this embodiment shown in FIGS. 21A and 21B.

The transistor 4010 included in the pixel portion 4002 is electricallyconnected to the display element to constitute part of a display panel.A variety of display elements can be used as the display element as longas display can be performed.

An example of a liquid crystal display device using a liquid crystalelement as a display element is illustrated in FIG. 21A. In FIG. 21A, aliquid crystal element 4013 which is the display element includes thefirst electrode layer 4030, a second electrode layer 4031, and a liquidcrystal layer 4008. Insulating films 4032 and 4033 serving as alignmentfilms are provided so that the liquid crystal layer 4008 is sandwichedtherebetween. The second electrode layer 4031 is provided on the secondsubstrate 4006 side, and the first electrode layer 4030 and the secondelectrode layer 4031 are stacked with the liquid crystal layer 4008provided therebetween.

A columnar spacer denoted by reference numeral 4035 is obtained byselective etching of an insulating film and is provided in order tocontrol the thickness (cell gap) of the liquid crystal layer 4008. Aspherical spacer may alternatively be used.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on a condition.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is not involved may be used. A blue phase is one ofliquid crystal phases, which is generated just before a cholestericphase changes into an isotropic phase while the temperature ofcholesteric liquid crystal is increased. Since the blue phase appearsonly in a narrow temperature range, a liquid crystal composition inwhich several weight percent or more of a chiral material is mixed isused for the liquid crystal layer in order to widen the temperaturerange. The liquid crystal composition which includes the liquid crystalexhibiting a blue phase and a chiral agent has a short response time,and has optical isotropy, which makes the alignment process unnecessaryand the viewing angle dependence small. In addition, since an alignmentfilm is not involved and thus rubbing treatment is unnecessary,electrostatic discharge damage caused by the rubbing treatment can beprevented, so that defects and damage of the liquid crystal displaydevice in the manufacturing process can be reduced. Thus, productivityof the liquid crystal display device can be increased. A transistorusing an oxide semiconductor film has a possibility that the electriccharacteristics may change significantly by the influence of staticelectricity to deviate from the designed range. Therefore, it is moreeffective to use a liquid crystal material exhibiting a blue phase forthe liquid crystal display device including the transistor using theoxide semiconductor film.

The inherent resistance of the liquid crystal material is greater thanor equal to 1×10⁹ Ω·cm, preferably greater than or equal to 1×10¹¹ Ω·cm,further preferably greater than or equal to 1×10¹² Ω·cm. The inherentresistance in this specification is measured at 20° C.

The magnitude of a storage capacitor provided in the liquid crystaldisplay device is set considering the leakage current of the transistorin the pixel portion or the like so that electric charge can be held fora predetermined period. The magnitude of the storage capacitor may beset considering the off-state current of the transistor or the like.Since the transistor including a highly purified crystalline oxidesemiconductor film is used, it is enough to provide a storage capacitorhaving a capacitance that is less than or equal to ⅓, preferably lessthan or equal to ⅕ of the liquid crystal capacitance of each pixel.

In the transistor used in this embodiment, which uses the highlypurified crystalline oxide semiconductor film, the current value in theoff state (off-state current value) is small. Accordingly, an electricalsignal such as an image signal can be retained for a longer period, andthe writing interval can be set longer in the power-on state.Accordingly, the frequency of refresh operation can be reduced, whichleads to a reduction in power consumption.

The field-effect mobility of the transistor using a highly purifiedcrystalline oxide semiconductor film used in this embodiment isrelatively high, which enables high-speed driving. For example, with useof such a transistor which can operate at high speed for a liquidcrystal display device, a switching transistor in a pixel portion and adriver transistor in a driver circuitry can be formed over onesubstrate. That is, a semiconductor device formed using a silicon waferor the like is not additionally needed as a driver circuitry, and thusthe number of components of the semiconductor device can be reduced. Inaddition, by using the transistor which can operate at high speed in thepixel portion, a high-quality image can be provided.

For the liquid crystal display device, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an optical compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, or the like can be used.

Further, a normally-black liquid crystal display device such as atransmissive liquid crystal display device utilizing a verticalalignment (VA) mode may be realized. Some examples can be given as thevertical alignment mode; for example, an MVA (multi-domain verticalalignment) mode, a PVA (patterned vertical alignment) mode, an ASV mode,or the like can be employed. Furthermore, one embodiment of the presentinvention can be applied to a VA liquid crystal display device. The VAliquid crystal display device has a kind of form in which alignment ofliquid crystal molecules of the liquid crystal display panel iscontrolled. In the VA liquid crystal display device, liquid crystalmolecules are aligned in a vertical direction with respect to the panelsurface when no voltage is applied. Further, it is possible to use amethod called domain multiplication or multi-domain design, in which apixel is divided into some regions (subpixels) and molecules are alignedin different directions in their respective regions.

In the display device, a black matrix (a light-blocking layer), anoptical member (an optical substrate) such as a polarizing member, aretardation member, or an anti-reflection member, and the like areprovided as appropriate. For example, circular polarization may beapplied with a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

As a display method in the pixel portion, a progressive method, aninterlace method, or the like can be employed. Further, color elementscontrolled in each pixel for color display are not limited to threecolors: R, G, and B (R, G, and B correspond to red, green, and blue,respectively). For example, R, G, B, and W (W corresponds to white); R,G, B, and one or more of yellow, cyan, magenta, and the like; or thelike can be used. Further, the size of the display region may bedifferent between respective dots of color elements. One embodiment ofthe present invention can be applied not only to a display device forcolor display, but also to a display device for monochrome display.

Further, a light-emitting element utilizing electroluminescence canalternatively be used as the display element in the display device.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether the light-emitting material is an organic compoundor an inorganic compound; in general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to thelight-emitting element, electrons and holes are separately injected froma pair of electrodes into a layer containing the light-emitting organiccompound, and current flows. Then, the carriers (electrons and holes)are recombined, and thus, the light-emitting organic compound isexcited. The light-emitting organic compound returns to a ground statefrom the excited state, thereby emitting light. Owing to such amechanism, this light-emitting element is referred to as acurrent-excitation light-emitting element.

Inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. The dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. On the other hand, the thin-film inorganicEL element has a structure where a light-emitting layer is sandwichedbetween dielectric layers, which is further sandwiched betweenelectrodes, and its light emission mechanism is localized type lightemission that utilizes inner-shell electron transition of metal ions. Anexample in which an organic EL element is used as the light-emittingelement is described here.

In order to extract light emitted from the light-emitting element, atleast one of the pair of electrodes has a light-transmitting property.The transistor and the light-emitting element are formed over asubstrate. The light-emitting element can have a top emission structurein which light emission is extracted through the surface opposite to thesubstrate; a bottom emission structure in which light emission isextracted through the surface on the substrate side; or a dual emissionstructure in which light emission is extracted through the side oppositeto the substrate and the substrate side.

FIG. 21B illustrates an example of a light-emitting device using alight-emitting element as a display element. A light-emitting element4513 which is the display element is electrically connected to thetransistor 4010 in the pixel portion 4002. The structure of thelight-emitting element 4513 is not limited to the stacked-layerstructure including the first electrode layer 4030, anelectroluminescent layer 4511, and the second electrode layer 4031,which is illustrated in FIG. 21B. The structure of the light-emittingelement 4513 can be changed as appropriate depending on the direction inwhich light is extracted from the light-emitting element 4513, or thelike.

A bank 4510 can be formed using an organic insulating material or aninorganic insulating material. It is particularly preferable that thebank 4510 be formed using a photosensitive resin material to have anopening over the first electrode layer 4030 so that a sidewall of theopening slopes with continuous curvature.

The electroluminescent layer 4511 consists of either a single layer or aplurality of layers stacked.

A protective film may be formed over the second electrode layer 4031 andthe bank 4510 in order to prevent entry of oxygen, hydrogen, moisture,carbon dioxide, or the like into the light-emitting element 4513. As theprotective film, a silicon nitride film, a silicon nitride oxide film, aDLC film, or the like can be formed. In addition, in a space which isconfined by the first substrate 4001, the second substrate 4006, and thesealant 4005, a filler 4514 is provided for sealing. It is preferablethat the panel be packaged (sealed) with a protective film (such as alaminate film or an ultraviolet curable resin film) or a cover memberwith high air-tightness and little degasification so that the panel isnot exposed to the outside air, in this manner.

As the filler 4514, an ultraviolet curable resin or a thermosettingresin can be used as well as an inert gas such as nitrogen or argon; forexample, polyvinyl chloride (PVC), an acrylic resin, a polyimide resin,an epoxy resin, a silicone resin, polyvinyl butyral (PVB), ethylenevinyl acetate (EVA), or the like can be used. For example, nitrogen isused for the filler.

In addition, if necessary, an optical film, such as a polarizing plate,a circularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter, may be provided as appropriate on a light-emissionsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by unevenness of the surface so as to reduce the glare can beperformed.

Further, an electronic paper in which electronic ink is driven can beprovided as the display device. The electronic paper is also called anelectrophoretic display device (an electrophoretic display) and isadvantageous in that it has the same level of readability as paper, ithas lower power consumption than other display devices, and it can bemade thin and lightweight.

The electrophoretic display device contains a plurality of microcapsulesdispersed in a solvent or a solute, each microcapsule containing firstparticles which are positively charged and second particles which arenegatively charged, in which by applying an electric field to themicrocapsules, the particles in the microcapsules move in oppositedirections to each other and only the color of the particles gatheringon one side is displayed, though there are various modes in theelectrophoretic display device. The first particles and the secondparticles each contain pigment and do not move without an electricfield. Further, the first particles and the second particles havedifferent colors (which may be colorless).

Thus, an electrophoretic display device is a display device thatutilizes a so-called dielectrophoretic effect by which a substancehaving a high dielectric constant moves to a high-electric field region.

A solution in which the above microcapsules are dispersed in a solventis called electronic ink. This electronic ink can be printed on asurface of glass, plastic, cloth, paper, or the like. Furthermore, byusing a color filter or particles that have a pigment, color display canalso be performed.

The first particle and the second particle in the microcapsules may eachbe formed of a single material selected from a conductive material, aninsulating material, a semiconductor material, a magnetic material, aliquid crystal material, a ferroelectric material, an electroluminescentmaterial, an electrochromic material, and a magnetophoretic material, ora composite material of any of these.

As the electronic paper, a display device using a twisting ball displaysystem can also be applied. The twisting ball display system refers to amethod in which spherical particles each colored in black and white arearranged between a first electrode layer and a second electrode layerwhich are electrode layers used for a display element, and a potentialdifference is generated between the first electrode layer and the secondelectrode layer to control orientation of the spherical particles, sothat display is performed.

In FIGS. 20A to 20C and FIGS. 21A and 21B, as each of the firstsubstrate 4001 and the second substrate 4006, a flexible substrate, forexample, a plastic substrate having a light-transmitting property or thelike can be used, as well as a glass substrate. As plastic, afiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride (PVF)film, a polyester film, or an acrylic resin film can be used. A sheetwith a structure in which an aluminum foil is sandwiched between PVFfilms or polyester films can be used as well.

In this embodiment, an aluminum oxide film is used as the insulatinglayer 4020. The insulating layer 4020 can be formed by a sputteringmethod or a plasma enhanced CVD method.

The aluminum oxide film provided as the insulating layer 4020 over anoxide semiconductor film has a high shielding effect (blocking effect)of blocking penetration of both oxygen and impurities such as hydrogenand moisture.

Therefore, in and after the manufacturing process, the aluminum oxidefilm functions as a protective film for preventing entry of an impuritysuch as hydrogen or moisture, which causes a change in characteristics,into the oxide semiconductor film and release of oxygen, which is a maincomponent material of the oxide semiconductor, from the oxidesemiconductor film.

The transistors 4010 and 4011 each include the crystalline oxidesemiconductor film obtained by recrystallization of an amorphous oxidesemiconductor film amorphized by adding oxygen into a crystalline oxidesemiconductor film. Since the heat treatment for crystallizing theamorphous oxide semiconductor film is performed in the state where theamorphous oxide semiconductor film is covered with the aluminum oxidefilm, oxygen can be prevented from being released from the amorphousoxide semiconductor film by the heat treatment. Thus, the resultingcrystalline oxide semiconductor film can maintain the amount of oxygencontained in the amorphous oxide semiconductor film, and therefore has aregion where the amount of oxygen is excess as compared to thestoichiometric oxide semiconductor in a crystalline state.

Therefore, the crystalline oxide semiconductor film has high puritybecause impurities such as hydrogen and moisture do not enter thecrystalline oxide semiconductor film, and has the region where theamount of oxygen is excess as compared to the stoichiometric oxidesemiconductor in a crystalline state because oxygen is prevented frombeing released therefrom. Accordingly, the crystalline oxidesemiconductor film enables a variation in the threshold voltage V_(th)of each of the transistors 4010 and 4011 and a shift of the thresholdvoltage (ΔV_(th)) due to an oxygen vacancy to be reduced.

The insulating layer 4021 serving as a planarizing insulating film canbe formed using an organic material having heat resistance, such asacrylic, polyimide, benzocyclobutene, polyamide, or epoxy. Other thansuch organic materials, it is also possible to use a low-dielectricconstant material (a low-k material), a siloxane-based resin, PSG(phosphosilicate glass), BPSG (borophosphosilicate glass), or the like.The insulating layer may be formed by stacking a plurality of insulatingfilms formed of these materials.

There is no particular limitation on the method for forming theinsulating layer 4021, and any of the following can be used depending ona material thereof: a method such as a sputtering method, an SOG method,spin coating, dipping, spray coating, or a droplet discharging method(e.g., an inkjet method), screen printing, or offset printing; a tool(equipment) such as doctor knife, roll coater, curtain coater, or knifecoater; or the like.

The display device displays an image by transmitting light from a lightsource or a display element. Therefore, it is necessary that thesubstrate and the thin films such as the insulating film and theconductive film provided for the pixel portion where light istransmitted have a light-transmitting property with respect to light inthe visible-light wavelength range.

On the other hand, the first electrode layer and the second electrodelayer (each of which is also called a pixel electrode layer, a commonelectrode layer, a counter electrode layer, or the like) for applyingvoltage to the display element each have either a light-transmittingproperty or a light-reflecting property, which depends on the directionin which light is extracted, the position where the electrode layer isprovided, the pattern structure of the electrode layer, and the like.

The first electrode layer 4030 and the second electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide, indium zinc oxide, indiumtin oxide to which silicon oxide is added, or graphene.

The first electrode layer 4030 and the second electrode layer 4031 eachcan also be formed using one or plural kinds selected from a metal suchas tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium(V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel(Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), orsilver (Ag); an alloy thereof; and a nitride thereof.

A conductive composition containing a conductive high molecule (alsocalled a conductive polymer) can also be used for any of the firstelectrode layer 4030 and the second electrode layer 4031. As theconductive high molecule, a so-called π-electron conjugated conductivepolymer can be used. For example, polyaniline or a derivative thereof,polypyrrole or a derivative thereof, polythiophene or a derivativethereof, a copolymer of two or more of aniline, pyrrole, and thiopheneor a derivative thereof can be given.

Further, since the transistor is likely to be broken by staticelectricity or the like, a protection circuit for protecting the drivercircuitry is preferably provided. The protection circuit is preferablyformed using a nonlinear element.

As described above, by applying any of the transistors described inEmbodiments 1 to 7, semiconductor devices having a variety of functionscan be provided.

Embodiment 9

A semiconductor device having an image sensing function for reading dataof an object can be manufactured with use of any of the transistorsdescribed in Embodiments 1 to 7.

FIG. 22A illustrates an example of a semiconductor device having animage sensing function. FIG. 22A is an equivalent circuit of a photosensor and FIG. 22B is a cross-sectional view illustrating part of thephoto sensor.

In a photodiode 602, one electrode is electrically connected to aphotodiode reset signal line 658, and the other electrode iselectrically connected to a gate of a transistor 640. One of a sourceand a drain of the transistor 640 is electrically connected to a photosensor reference signal line 672, and the other of the source and thedrain thereof is electrically connected to one of a source and a drainof a transistor 656. A gate of the transistor 656 is electricallyconnected to a gate signal line 659, and the other of the source and thedrain thereof is electrically connected to a photo sensor output signalline 671.

In the circuit diagram in this specification, a transistor using anoxide semiconductor film is put down with a symbol “OS” so as to beclearly identified as a transistor using an oxide semiconductor film. InFIG. 22A, the transistor 640 and the transistor 656 are transistors eachusing a crystalline oxide semiconductor film, such as the transistor 440described in Embodiment 1.

FIG. 22B is a cross-sectional view of the photodiode 602 and thetransistor 640 in the photo sensor. The photodiode 602 functioning as asensor and the transistor 640 are provided over a substrate 601 (a TFTsubstrate) having an insulating surface. A substrate 613 is providedover the photodiode 602 and the transistor 640 with an adhesive layer608 provided therebetween.

An insulating layer 631, an interlayer insulating layer 633, and aninterlayer insulating layer 634 are provided over the transistor 640.The photodiode 602 is provided over the interlayer insulating layer 633.In the photodiode 602, a first semiconductor layer 606 a, a secondsemiconductor layer 606 b, and a third semiconductor layer 606 c aresequentially stacked from the interlayer insulating layer 633 side,between an electrode layer 641 formed over the interlayer insulatinglayer 633 and an electrode layer 642 formed over the interlayerinsulating layer 634.

The electrode layer 641 is electrically connected to a conductive layer643 provided for the interlayer insulating layer 634, and the electrodelayer 642 is electrically connected to a conductive layer 645 throughthe electrode layer 641. The conductive layer 645 is electricallyconnected to a gate electrode layer of the transistor 640, and thephotodiode 602 is electrically connected to the transistor 640.

Described in this embodiment is a pin photodiode in which asemiconductor layer having a p-type conductivity as the firstsemiconductor layer 606 a, a high-resistance semiconductor layer (i-typesemiconductor layer) as the second semiconductor layer 606 b, and asemiconductor layer having an n-type conductivity as the thirdsemiconductor layer 606 c are stacked.

The first semiconductor layer 606 a is a p-type semiconductor layer andcan be formed using an amorphous silicon film containing an impurityelement imparting a p-type conductivity. The first semiconductor layer606 a is formed by a plasma enhanced CVD method with use of asemiconductor source gas containing an impurity element belonging toGroup 13 (such as boron (B)). As the semiconductor source gas, silane(SiH₄) may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄,or the like may be used. Further alternatively, an amorphous siliconfilm which does not contain an impurity element may be formed, and then,an impurity element may be introduced to the amorphous silicon film withuse of a diffusion method or an ion implantation method. Heating or thelike may be performed thereon after adding the impurity element by anion implantation method or the like in order to diffuse the impurityelement. In that case, as the method for forming the amorphous siliconfilm, an LPCVD method, a chemical vapor deposition method, a sputteringmethod, or the like may be used. The first semiconductor layer 606 a ispreferably formed to have a thickness greater than or equal to 10 nm andless than or equal to 50 nm.

The second semiconductor layer 606 b is an i-type semiconductor layer(intrinsic semiconductor layer) and is formed using an amorphous siliconfilm. As for formation of the second semiconductor layer 606 b, anamorphous silicon film is formed with use of a semiconductor source gasby a plasma enhanced CVD method. As the semiconductor source gas, silane(SiH₄) may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄,or the like may be used. The second semiconductor layer 606 b may bealternatively formed by an LPCVD method, a vapor deposition method, asputtering method, or the like. The second semiconductor layer 606 b ispreferably formed to have a thickness greater than or equal to 200 nmand less than or equal to 1000 nm.

The third semiconductor layer 606 c is an n-type semiconductor layer andis formed using an amorphous silicon film containing an impurity elementimparting an n-type conductivity. The third semiconductor layer 606 c isformed by a plasma enhanced CVD method with use of a semiconductorsource gas containing an impurity element belonging to Group 15 (e.g.,phosphorus (P)). As the semiconductor source gas, silane (SiH₄) may beused. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the likemay be used. Further alternatively, an amorphous silicon film which doesnot contain an impurity element may be formed, and then, an impurityelement may be introduced to the amorphous silicon film with use of adiffusion method or an ion implantation method. Heating or the like maybe performed thereon after adding the impurity element by an ionimplantation method or the like in order to diffuse the impurityelement. In that case, as the method for forming the amorphous siliconfilm, an LPCVD method, a chemical vapor deposition method, a sputteringmethod, or the like may be used. The third semiconductor layer 606 c ispreferably formed to have a thickness greater than or equal to 20 nm andless than or equal to 200 nm.

Any of the first semiconductor layer 606 a, the second semiconductorlayer 606 b, and the third semiconductor layer 606 c is not necessarilyformed using an amorphous semiconductor, and may be formed using apolycrystalline semiconductor, or a microcrystalline semiconductor(semi-amorphous semiconductor: SAS).

The microcrystalline semiconductor belongs to a metastable state of anintermediate between amorphous and single crystalline considering Gibbsfree energy. That is, the microcrystalline semiconductor film is asemiconductor having a third state which is stable in terms of freeenergy and has a short range order and lattice distortion. Columnar-likeor needle-like crystals grow in a normal direction with respect to asubstrate top surface. The Raman spectrum of microcrystalline silicon,which is a typical example of a microcrystalline semiconductor, islocated in lower wave numbers than 520 cm⁻¹, which represents a peak ofthe Raman spectrum of single crystal silicon. That is, the peak of theRaman spectrum of the microcrystalline silicon exists between 520 cm⁻¹which represents single crystal silicon and 480 cm⁻¹ which representsamorphous silicon. In addition, microcrystalline silicon containshydrogen or halogen of at least 1 atomic percent or more in order toterminate a dangling bond. Moreover, a rare gas element such as helium,argon, krypton, or neon may be contained to further promote the latticedistortion, whereby the stability is increased and thus a favorablemicrocrystalline semiconductor can be obtained.

The microcrystalline semiconductor film can be formed by ahigh-frequency plasma CVD method with a frequency of several tens ofmegahertz to several hundreds of megahertz or using a microwave plasmaCVD apparatus with a frequency of 1 GHz or more. Typically, themicrocrystalline semiconductor film can be formed using a compoundcontaining silicon such as SiH₄, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, or SiF₄,which is diluted with hydrogen. The microcrystalline semiconductor filmcan also be formed with dilution with one or plural kinds of rare gaselements selected from helium, argon, krypton, and neon in addition tothe compound containing silicon (e.g., silicon hydride) and hydrogen. Inthose cases, the flow ratio of hydrogen to the compound containingsilicon (e.g., silicon hydride) is 5:1 to 200:1, preferably 50:1 to150:1, further preferably 100:1. Further, a carbide gas such as CH₄ orC₂H₆, a germanium gas such as GeH₄ or GeF₄, F₂, or the like may be mixedinto the gas containing silicon.

In addition, since the mobility of holes generated by the photoelectriceffect is lower than that of electrons, such a pin photodiode exhibitsbetter characteristics when a surface on the p-type semiconductor layerside is used as a light-receiving plane. Here, an example in which lightreceived by the photodiode 602 from a surface of the substrate 601, overwhich the pin photodiode is formed, is converted into electric signalsis described. Further, light approaching the semiconductor layer havingthe conductivity type opposite from that of the semiconductor layer onthe light-receiving plane is disturbance light; therefore, alight-blocking conductive film is preferably used for the electrodelayer on the semiconductor layer having the opposite conductivity type.The surface on the n-type semiconductor layer side can alternatively beused as the light-receiving plane.

The interlayer insulating layer 633, 634 can be formed using aninsulating material by a sputtering method, a plasma enhanced CVDmethod, an SOG method, spin coating, dipping, spray coating, or adroplet discharge method (e.g., an inkjet method), screen printing,offset printing, or the like, or a tool (equipment) such as a doctorknife, a roll coater, a curtain coater, or a knife coater, depending ona material.

In this embodiment, an aluminum oxide film is used as the insulatinglayer 631. The insulating layer 631 can be formed by a sputtering methodor a plasma enhanced CVD method.

The aluminum oxide film provided as the insulating layer 631 over anoxide semiconductor film has a high shielding effect (blocking effect)of blocking penetration of both oxygen and impurities such as hydrogenand moisture.

Therefore, in and after the manufacturing process, the aluminum oxidefilm functions as a protective film for preventing entry of an impuritysuch as hydrogen or moisture, which causes a change in characteristics,into the oxide semiconductor film and release of oxygen, which is a maincomponent material of the oxide semiconductor, from the oxidesemiconductor film.

The transistor 640 includes the crystalline oxide semiconductor filmobtained by recrystallization of an amorphous oxide semiconductor filmamorphized by adding oxygen into a crystalline oxide semiconductor film.Since the heat treatment for crystallizing the amorphous oxidesemiconductor film is performed in the state where the amorphous oxidesemiconductor film is covered with the aluminum oxide film, oxygen canbe prevented from being released from the amorphous oxide semiconductorfilm by the heat treatment. Thus, the resulting crystalline oxidesemiconductor film can maintain the amount of oxygen contained in theamorphous oxide semiconductor film, and therefore has a region where theamount of oxygen is excess as compared to the stoichiometric oxidesemiconductor in a crystalline state.

Therefore, the crystalline oxide semiconductor film has high puritybecause impurities such as hydrogen and moisture do not enter thecrystalline oxide semiconductor film, and has the region where theamount of oxygen is excess as compared to the stoichiometric oxidesemiconductor in a crystalline state because oxygen is prevented frombeing released therefrom. Accordingly, the crystalline oxidesemiconductor film enables a variation in the threshold voltage V_(th)of the transistor 640 and a shift of the threshold voltage (ΔV_(th)) dueto an oxygen vacancy to be reduced.

For reduction of the surface roughness, an insulating layer functioningas a planarizing insulating film is preferably used as the interlayerinsulating layer 633, 634. For the interlayer insulating layer 633, 634,an organic insulating material having heat resistance such as polyimide,acrylic resin, benzocyclobutene resin, polyamide, or epoxy resin can beused. Other than such organic insulating materials, it is also possibleto use a single layer or stacked layers of a low-dielectric constantmaterial (a low-k material), a siloxane-based resin, phosphosilicateglass (PSG), borophosphosilicate glass (BPSG), or the like.

With detection of light entering the photodiode 602, data on an objectto be detected can be read. A light source such as a backlight can beused at the time of reading data on the object.

The transistor including the highly purified crystalline oxidesemiconductor film containing an excess of oxygen that repairs an oxygenvacancy as described above has less change in electric characteristicsand thus is electrically stable. Accordingly, with the transistor, ahighly reliable semiconductor device can be provided.

This embodiment can be implemented combining with any structuredescribed in the other embodiments as appropriate.

Embodiment 10

The transistor an example of which is described in any of Embodiments 1to 7 can be suitably used for a semiconductor device including anintegrated circuit in which a plurality of transistors is stacked. Inthis embodiment, a memory medium (memory element) is described as anexample of the semiconductor device.

In this embodiment, a semiconductor device including a transistor 140that is a first transistor formed using a single crystal semiconductorsubstrate, and a transistor 162 that is a second transistor formed usinga semiconductor film above the transistor 140 with an insulating layerprovided therebetween, is manufactured. The transistor an example ofwhich is described in any of Embodiments 1 to 7 can be suitably used asthe transistor 162. Described in this embodiment is an example in whicha transistor having a structure similar to that of the transistor 440described in Embodiment 1 is used as the transistor 162.

Semiconductor materials and structures of the transistor 140 and thetransistor 162, which are stacked, may be the same as or different fromeach other. Described in this embodiment is an example in which atransistor with a suitable material and a suitable structure is used ineach circuit of a memory medium (memory element).

FIGS. 19A and 19B illustrate an example of a structure of asemiconductor device. FIG. 19A illustrates a cross section of thesemiconductor device, and FIG. 19B illustrates a plan view of thesemiconductor device. Here, FIG. 19A corresponds to the cross sectionalong line C1-C2 and line D1-D2 in FIG. 19B. FIG. 19C illustrates anexample of a circuit diagram of the semiconductor device when being usedas a memory element. The semiconductor device illustrated in FIGS. 19Aand 19B includes the transistor 140 using a first semiconductor materialin a lower portion, and the transistor 162 using a second semiconductormaterial in an upper portion. In this embodiment, the firstsemiconductor material is a semiconductor material other than an oxidesemiconductor, and the second semiconductor material is an oxidesemiconductor. As the semiconductor material other than an oxidesemiconductor, for example, silicon, germanium, silicon germanium,silicon carbide, gallium arsenide, or the like can be used, and a singlecrystal semiconductor is preferably used. An organic semiconductormaterial or the like may also be used. A transistor using such asemiconductor material other than an oxide semiconductor can operate athigh speed easily. On the other hand, a transistor using an oxidesemiconductor can retain electric charge for a long time owing to itscharacteristics.

A method for manufacturing the semiconductor device shown in FIGS. 19Ato 19C is described with reference to FIGS. 19A to 19C.

The transistor 140 includes a channel formation region 116 provided in asubstrate 185 including a semiconductor material (e.g., silicon),impurity regions 120 provided so that the channel formation region 116is sandwiched therebetween, metal compound regions 124 in contact withthe impurity regions 120, a gate insulating layer 108 provided over thechannel formation region 116, and a gate electrode 110 provided over thegate insulating layer 108.

As the substrate 185 including a semiconductor material, a singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate comprised of silicon, silicon carbide, or the like; a compoundsemiconductor substrate of silicon germanium or the like; an SOIsubstrate; or the like can be used. Although the “SOI substrate”generally means a substrate in which a silicon semiconductor layer isprovided on an insulating surface, the “SOI substrate” in thisspecification and the like also encompasses a substrate in which asemiconductor layer formed of a material other than silicon is providedon an insulating surface. That is, the semiconductor layer included inthe “SOI substrate” is not limited to a silicon semiconductor layer.Moreover, the SOI substrate encompasses a substrate having a structurein which a semiconductor layer is provided over an insulating substratesuch as a glass substrate, with an insulating layer providedtherebetween.

The SOI substrate can be manufactured by any of the following methods:oxygen ions are added in a mirror-polished wafer, and then the wafer isheated at a high temperature(s) to form an oxidized layer at apredetermined depth from a top surface of the wafer and eliminatedefects generated in the superficial layer; a semiconductor substrate isseparated by utilizing the growth of microvoids, which are formed byhydrogen ion irradiation, by heat treatment; a single crystalsemiconductor layer is formed on an insulating surface by crystalgrowth.

For example, ions are added through one surface of a single crystalsemiconductor substrate to form an embrittlement layer at a certaindepth from the surface of the single crystal semiconductor substrate,and an insulating layer is formed over one of the single crystalsemiconductor substrate and an element substrate. Heat treatment isperformed in a state where the single crystal semiconductor substrateand the element substrate are overlapped with each other with theinsulating layer provided therebetween, so that a crack is generated inthe embrittlement layer, whereby a single crystal semiconductor layer isformed of the single crystal semiconductor substrate, as a semiconductorlayer. An SOI substrate formed by the above-described method can also besuitably used.

An element isolation insulating layer 106 is provided over the substrate185 so as to surround the transistor 140. Note that it is preferablethat the transistor 140 does not have a sidewall insulating layer asillustrated in FIGS. 19A to 19C in order to achieve high integration. Onthe other hand, when importance is put on the characteristics of thetransistor 140, a sidewall insulating layer may be provided on the sidesurface of the gate electrode 110 to form another impurity region whoseimpurity concentration is different from an impurity concentration ofthe impurity region 120 in the impurity region 120.

The transistor 140 using a single crystal semiconductor substrate canoperate at high speed. Thus, with use of the transistor as a readingtransistor, data can be read at a high speed. Two insulating layers areformed so as to cover the transistor 140. As treatment before thetransistor 162 and a capacitor 164 are formed, CMP treatment isperformed on the two insulating layers, whereby planarized insulatinglayers 128 and 130 are formed and an upper surface of the gate electrode110 is exposed.

As the insulating layer 128, 130, typically, an inorganic insulatingfilm such as a silicon oxide film, a silicon oxynitride film, analuminum oxide film, an aluminum oxynitride film, a silicon nitridefilm, an aluminum nitride film, a silicon nitride oxide film, or analuminum nitride oxide film can be used. The insulating layer 128, 130can be formed by a plasma enhanced CVD method, a sputtering method, orthe like.

Alternatively, an organic material such as polyimide, an acrylic resin,or a benzocyclobutene resin can be used. Other than such organicmaterials, it is also possible to use a low-dielectric constant material(a low-k material) or the like. In the case of using an organicmaterial, a wet process such as a spin coating method or a printingmethod may be applied to form the insulating layer 128, 130.

In the insulating layer 130, a silicon oxide film is used as a film tobe in contact with the semiconductor layer.

In this embodiment, a 50-nm-thick silicon oxynitride film is formed by asputtering method as the insulating layer 128, and a 550-nm-thicksilicon oxide film is formed by a sputtering method as the insulatinglayer 130.

A semiconductor film is formed over the insulating layer 130 planarizedsufficiently by the CMP treatment. In this embodiment, a crystallineoxide semiconductor film is formed as the semiconductor film by asputtering method using an In—Ga—Zn—O-based oxide target so as toinclude a region containing excess oxygen as compared to thestoichiometric composition ratio of the oxide semiconductor in acrystalline state.

Next, the crystalline oxide semiconductor film is selectively etched toform an island-shaped crystalline oxide semiconductor film. Oxygen isadded into the crystalline oxide semiconductor film, whereby anamorphous oxide semiconductor film is obtained. Over the amorphous oxidesemiconductor film, a gate insulating layer 146, a gate electrode layer148, and sidewall insulating layers 136 a and 136 b are formed.

As the gate insulating layer 146, a silicon oxide film, a siliconnitride film, a silicon oxynitride film, a silicon nitride oxide film,an aluminum oxide film, an aluminum nitride film, an aluminum oxynitridefilm, an aluminum nitride oxide film, a hafnium oxide film, or a galliumoxide film can be formed by a plasma enhanced CVD method, a sputteringmethod, or the like.

The gate electrode layer 148 can be formed by forming a conductive layerover the gate insulating layer 146 and selectively etching theconductive layer.

Then, a conductive layer is formed over the gate electrode 110, theinsulating layer 128, the insulating layer 130, and the like and isselectively etched, whereby a source and drain electrodes 142 a and 142b are formed.

The conductive layer can be formed by a PVD method such as a sputteringmethod, a CVD method such as a plasma enhanced CVD method. Further, as amaterial of the conductive layer, an element selected from Al, Cr, Cu,Ta, Ti, Mo, and W, an alloy including the above element as itscomponent, or the like can be used. Any of Mn, Mg, Zr, Be, Nd, and Sc,or a material including any of these in combination may also be used.

The conductive layer has either a single-layer structure or astacked-layer structure including two or more layers. For example, theconductive layer can have a single-layer structure of a titanium film ora titanium nitride film, a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a two-layer structure in which a titaniumfilm is stacked over a titanium nitride film, or a three-layer structurein which a titanium film, an aluminum film, and a titanium film arestacked in this order. In the case where the conductive layer has asingle-layer structure of a titanium film or a titanium nitride film,there is an advantage of high processability of the conductive layerinto the source and drain electrodes 142 a and 142 b to provide taperedshapes.

Next, an insulating layer 150 including an aluminum oxide film is formedover the amorphous oxide semiconductor film, the gate insulating layer146, the gate electrode layer 148, and the sidewall insulating layers136 a and 136 b. In the case where the insulating layer 150 has astacked-layer structure, a stack of the aluminum oxide film and asilicon oxide film, a silicon nitride film, a silicon oxynitride film, asilicon nitride oxide film, an aluminum nitride film, an aluminumoxynitride film, an aluminum nitride oxide film, a hafnium oxide film,or a gallium oxide film may be formed by a plasma enhanced CVD method, asputtering method, or the like.

Next, heat treatment is performed on the amorphous oxide semiconductorfilm to amorphize at least part of the amorphous oxide semiconductorfilm, so that a crystalline oxide semiconductor film 144 including acrystal having a c-axis which is substantially perpendicular to a topsurface of the crystalline oxide semiconductor film 144 is formed.

The aluminum oxide film provided as the insulating layer 150 over theoxide semiconductor film has a high shielding effect (blocking effect)of blocking penetration of both oxygen and impurities such as hydrogenand moisture.

Therefore, in and after the manufacturing process, the aluminum oxidefilm functions as a protective film for preventing entry of an impuritysuch as hydrogen or moisture, which causes a change in characteristics,into the oxide semiconductor film and release of oxygen, which is a maincomponent material of the oxide semiconductor, from the oxidesemiconductor film.

Since the heat treatment for crystallizing the amorphous oxidesemiconductor film is performed in the state where the amorphous oxidesemiconductor film is covered with the aluminum oxide film provided asthe insulating layer 150, oxygen can be prevented from being releasedfrom the amorphous oxide semiconductor film by the heat treatment. Thus,the resulting crystalline oxide semiconductor film 144 can maintain theamount of oxygen contained in the amorphous oxide semiconductor film,and therefore has a region where the amount of oxygen is excess ascompared to the stoichiometric oxide semiconductor in a crystallinestate.

Therefore, the crystalline oxide semiconductor film 144 has high puritybecause impurities such as hydrogen and moisture do not enter thecrystalline oxide semiconductor film 144, and has the region where theamount of oxygen is excess as compared to the stoichiometric oxidesemiconductor in a crystalline state because oxygen is prevented frombeing released therefrom. Accordingly, the crystalline oxidesemiconductor film 144 enables a variation in the threshold voltageV_(th) of the transistor 162 and a shift of the threshold voltage(ΔV_(th)) due to an oxygen vacancy to be reduced.

The heat treatment for crystallizing at least part of the amorphousoxide semiconductor film is performed at a temperature(s) higher than orequal to 250° C. and lower than or equal to 700° C., preferably higherthan or equal to 400° C., further preferably higher than or equal to500° C., still further preferably higher than or equal to 550° C.

Over the insulating layer 150, an electrode layer 153 is formed in aregion which overlaps with the source or drain electrode 142 a.

Next, an insulating layer 152 is formed over the transistor 162 and theinsulating layer 150. The insulating layer 152 can be formed by asputtering method, a CVD method, or the like. The insulating layer 152can be formed using a material including an inorganic insulatingmaterial such as silicon oxide, silicon oxynitride, silicon nitride,hafnium oxide, or aluminum oxide.

Next, an opening that reaches the source or drain electrode 142 b isformed in the gate insulating layer 146, the insulating layer 150, andthe insulating layer 152. The opening is formed by selective etchingusing a mask or the like.

After that, a wiring 156 is formed in the opening to be in contact withthe source or drain electrode 142 b. The connection point of the sourceor drain electrode 142 b and the wiring 156 is not shown in FIG. 19A.

The wiring 156 is formed in such a manner that a conductive layer isformed by a PVD method such as a sputtering method or a CVD method suchas a plasma enhanced CVD method and then is etched. Further, as amaterial of the conductive layer, an element selected from Al, Cr, Cu,Ta, Ti, Mo, and W, an alloy including the above element as itscomponent, or the like can be used. Any of Mn, Mg, Zr, Be, Nd, and Sc,or a material including any of these in combination may also be used.The details are the same as those of the source or drain electrode 142a.

Through the above steps, the transistor 162 and the capacitor 164 areformed. The transistor 162 includes the highly purified crystallineoxide semiconductor film 144 containing excess oxygen which repairs anoxygen vacancy. Therefore, change in the electric characteristics of thetransistor 162 is suppressed, and the transistor 162 is electricallystable. The capacitor 164 includes the source or drain electrode 142 a,the crystalline oxide semiconductor film 144, the gate insulating layer146, and the electrode layer 153.

The crystalline oxide semiconductor film 144 and the gate insulatinglayer 146 are stacked in the capacitor 164 in FIG. 19A, whereby theinsulation between the source or drain electrode 142 a and the electrodelayer 153 can be sufficiently provided. It is needless to say that thecrystalline oxide semiconductor film 144 can be omitted in the capacitor164 in order to provide sufficient capacitance. An insulating layer maybe included in the capacitor 164. The capacitor 164 can be omitted inthe case where a capacitor is not needed.

FIG. 19C illustrates an example of a circuit diagram of thesemiconductor device when used as a memory element. In FIG. 19C, one ofa source electrode and a drain electrode of the transistor 162, oneelectrode of the capacitor 164, and a gate electrode of the transistor140 are electrically connected to one another. A first wiring (1st Line,also referred to as a source line) is electrically connected to a sourceelectrode of the transistor 140, and a second wiring (2nd Line, alsoreferred to as a bit line) is electrically connected to a drainelectrode of the transistor 140. Further, a third wiring (3rd Line, alsoreferred to as a first signal line) is electrically connected to theother of the source electrode and the drain electrode of the transistor162, and a fourth wiring (4th Line, also referred to as a second signalline) is electrically connected to a gate electrode of the transistor162. A fifth wiring (5th line, also referred to as a word line) iselectrically connected to the other electrode of the capacitor 164.

The transistor 162 using an oxide semiconductor has extremely smalloff-state current; therefore, when the transistor 162 is in an offstate, the potential of a node (hereinafter, a node FG) where the one ofthe source electrode and drain electrode of the transistor 162, the oneelectrode of the capacitor 164, and the gate electrode of the transistor140 are electrically connected to one another can be retained for anextremely long time. The capacitor 164 facilitates retention of electriccharge given to the node FG and reading of the data.

When data is stored in the semiconductor device (in data writing), thepotential of the fourth wiring is set to a potential at which thetransistor 162 is turned on, whereby the transistor 162 is turned on.Thus, the potential of the third wiring is applied to the node FG and apredetermined amount of electric charge is accumulated in the node FGHere, electric charge for applying either two different potential levels(hereinafter referred to as low-level charge and high-level charge) isgiven to the node FG. After that, the potential of the fourth wiring ischanged to a potential at which the transistor 162 is turned off,whereby the transistor 162 is turned off. Consequently, the node FG ismade into a floating state to retain the predetermined amount ofelectric charge in the node FG In this manner, the predetermined amountof charge is accumulated and retained in the node FG, whereby data canbe stored in the memory cell.

Since the off-state current of the transistor 162 is extremely small,the charge supplied to the node FG is retained for a long period.Accordingly, a refresh operation can be omitted or the frequency of therefresh operation can be drastically reduced, which leads to asufficient reduction in power consumption. Further, stored data can beretained for a long time even with no power supply.

When stored data is read out (in data reading), while a certainpotential (a fixed potential) is applied to the first wiring, anappropriate potential (a read-out potential) is applied to the fifthwiring, whereby the transistor 140 changes its state depending on theamount of charge retained in the node FG. This is because in general,when the transistor 140 is an n-channel transistor, an apparentthreshold value V_(th) _(_) _(H) of the transistor 140 in the case wherethe high-level charge is retained in the node FG is lower than anapparent threshold value V_(th) _(_) _(L) of the transistor 140 in thecase where the low-level charge is retained in the node FG. Here, eachapparent threshold voltage refers to the potential of the fifth wiring,which is needed to turn on the transistor 140. Thus, by setting thepotential of the fifth wiring to a potential V₀ which is between V_(th)_(_) _(H) and V_(th) _(_) _(L), electric charge retained in the node FGcan be determined. For example, in the case where the high-levelelectric charge is given in data writing, the transistor 140 is turnedon when the potential of the fifth wiring is V₀ (>V_(th) _(_) _(H)). Incontrast, in the case where the low-level electric charge is given indata writing, the transistor 140 remains in its off state even when thepotential of the fifth wiring is V₀ (<V_(th) _(_) _(L)). In this manner,the potential of the fifth wiring is controlled and whether thetransistor 140 is in the on state or the off state (the potential of thesecond wiring) is read out, whereby stored data can be read out.

Further, when stored data is rewritten, a new potential is applied tothe node FG that retains the predetermined amount of charge given in theabove writing, so that the charge of the new data is retained in thenode FG. Specifically, the potential of the fourth wiring is set to thepotential at which the transistor 162 is turned on, whereby thetransistor 162 is turned on. The potential of the third wiring (thepotential of new data) is consequently applied to the node FG, and thepredetermined amount of charge is accumulated in the node FG. Afterthat, the potential of the fourth wiring is changed to the potential atwhich the transistor 162 is turned off, whereby the transistor 162 isturned off, so that the charge of the new data is retained in the nodeFG. In other words, on the state where the predetermined amount ofcharge given in the first writing is retained in the node FG, anoperation (a second writing) similar to the first writing is performed,whereby the stored data can be overwritten.

The off-state current of the transistor 162 described in this embodimentcan be sufficiently reduced by using the highly purified oxidesemiconductor film containing excess oxygen as the crystalline oxidesemiconductor film 144. Further, with such a transistor, a semiconductordevice in which stored data can be retained for an extremely long timecan be provided.

As described above, a transistor which includes a highly purifiedcrystalline oxide semiconductor film containing excess oxygen whichrepairs an oxygen vacancy has less change in electric characteristicsand is electrically stable. Thus, with the transistor, a highly reliablesemiconductor device can be provided.

The structures, methods, and the like described in this embodiment canbe used in appropriate combination with any of the structures, methods,and the like described in the other embodiments.

Embodiment 11

A semiconductor device disclosed in this specification can be applied toa variety of electronic devices (including game machines). Examples ofelectronic devices are a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.Examples of an electronic device including the semiconductor devicedescribed in the above embodiment are described below.

FIG. 23A illustrates a laptop personal computer, which includes a mainbody 3001, a housing 3002, a display portion 3003, a keyboard 3004, andthe like. Application of the semiconductor device described in any ofEmbodiments 1 to 10 to the display portion 3003 enables a highlyreliable laptop personal computer to be provided.

FIG. 23B is a personal digital assistant (PDA), which includes a mainbody 3021 provided with a display portion 3023, an external interface3025, operation buttons 3024, and the like. A stylus 3022 is included asan accessory for operation. Application of the semiconductor devicedescribed in any of Embodiments 1 to 10 to the display portion 3023enables a highly reliable personal digital assistant (PDA) to beprovided.

FIG. 23C illustrates an example of an e-book reader. For example, ane-book reader 2700 includes two housings, a housing 2701 and a housing2703. The housing 2701 and the housing 2703 are combined with a hinge2711 so that the e-book reader 2700 can be opened and closed with thehinge 2711 as an axis. With such a structure, the e-book reader 2700 canoperate like a paper book.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. One screen image ordifferent screen images may be displayed on the display portion 2705 andthe display portion 2707. In the structure where different screen imagesare displayed on the display portion 2705 and the display portion 2707,for example, text can be displayed on the right display portion (thedisplay portion 2705 in FIG. 23C) and images can be displayed on theleft display portion (the display portion 2707 in FIG. 23C). By applyingthe semiconductor device described in any of Embodiments 1 to 10 to thedisplay portion 2705, 2707, the e-book reader 2700 with high reliabilitycan be provided. In the case of using a transflective or reflectiveliquid crystal display device as the display portion 2705, the e-bookreader may be used in a comparatively bright environment; therefore, asolar cell may be provided so that power generation by the solar celland charge by a battery can be performed. When a lithium ion battery isused as the battery, there are advantages of downsizing and the like.

FIG. 23C illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, an operation key 2723, a speaker2725, and the like. With the operation key 2723, pages can be turned. Akeyboard, a pointing device, or the like may also be provided on thesame plane of the housing, as the display portion. Furthermore, anexternal connection terminal (an earphone terminal, a USB terminal, orthe like), a recording medium insertion portion, and the like may beprovided on the back surface or the side surface of the housing. Thee-book reader 2700 may have a function of an electronic dictionary.

The e-book reader 2700 may have a configuration capable of wirelesslytransmitting and receiving data. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an electronicbook server.

FIG. 23D illustrates a mobile phone, which includes two housings, ahousing 2800 and a housing 2801. The housing 2801 is provided with adisplay panel 2802, a speaker 2803, a microphone 2804, a pointing device2806, a camera lens 2807, an external connection terminal 2808, and thelike. The housing 2800 is provided with a solar cell 2810 for chargingthe mobile phone, an external memory slot 2811, and the like. Further,an antenna is incorporated in the housing 2801. Application of thesemiconductor device described in any of Embodiments 1 to 10 to thedisplay panel 2802 enables a highly reliable mobile phone to beprovided.

Further, the display panel 2802 is provided with a touch panel. Aplurality of operation keys 2805 which are displayed as images are shownby dashed lines in FIG. 23D. A boosting circuit by which a voltageoutput from the solar cell 2810 is increased to be sufficiently high foreach circuit is also equipped.

In the display panel 2802, the display direction can be appropriatelychanged depending on a usage pattern. Further, the display device isprovided with the camera lens 2807 on the same plane as the displaypanel 2802, which enables videophone calls. The speaker 2803 and themicrophone 2804 can be used for videophone calls, recording and playingsound, and the like as well as voice calls. Further, the housings 2800and 2801 in a state where they are developed as illustrated in FIG. 23Dcan shift by sliding to a state where one is overlapped with the other;therefore, the size of the mobile phone can be reduced, which makes themobile phone suitable for being carried.

The external connection terminal 2808 can be connected to an AC adapterand various types of cables such as a USB cable, which enables chargingand data communication with a personal computer. Further, a large amountof data can be stored and carried by a storage medium inserted into theexternal memory slot 2811.

In addition to the above functions, an infrared communication function,a television reception function, or the like may be equipped.

FIG. 23E illustrates a digital video camera which includes a main body3051, a display portion A 3057, an eyepiece 3053, an operation switch3054, a display portion B 3055, a battery 3056, and the like.Application of the semiconductor device described in any of Embodiments1 to 10 to the display portion A 3057 and/or the display portion B 3055enables a highly reliable digital video camera to be provided.

FIG. 23F illustrates an example of a television set. In a television set9600, a display portion 9603 is incorporated in a housing 9601. Imagescan be displayed on the display portion 9603. Here, the housing 9601 issupported by a stand 9605. Application of the semiconductor devicedescribed in any of Embodiments 1 to 10 to the display portion 9603enables the reliability of the television set 9600 to be increased.

The television set 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

The television set 9600 is provided with a receiver, a modem, and thelike. With use of the receiver, general television broadcasting can bereceived. Moreover, the display device can be connected to acommunication network with or without wires via the modem, wherebyone-way (from sender to receiver) or two-way (between sender andreceiver or between receivers) data communication can be performed.

This embodiment can be implemented combining with any structuredescribed in the other embodiments as appropriate.

Example 1

In this example, properties of an aluminum oxide film in terms of abarrier film in a semiconductor device according to one embodiment ofthe present invention were evaluated. As the evaluation method,secondary ion mass spectrometry (SIMS) and thermal desorptionspectrometry (TDS) were used.

First, results of evaluation on oxygen diffusion in an aluminum oxidefilm (Example Sample B) according to SIMS are shown. For comparison,oxygen diffusion in a silicon oxide film (Comparison Example Sample B)and in an In—Ga—Zn—O film (Comparison Example Sample C) was alsoevaluated.

In each of Example Sample B, Comparison Example Sample B, and ComparisonExample Sample C, a 300-nm-thick silicon oxide film (¹⁸O) was formedusing oxygen (¹⁸O) over a silicon substrate by a sputtering method. Inthis example, in order to discriminate oxygen which is diffused from theoutside to each sample film after formation of the sample film fromoxygen (¹⁶O) that is a component of the sample film, the silicon oxidefilm (¹⁸O) using oxygen (¹⁸O) that is an isotope of oxygen (¹⁶O) that isa component of the sample film, was provided in contact with each samplefilm as a diffusion source of oxygen (¹⁸O).

The deposition conditions of the silicon oxide film (¹⁸O) were asfollows: a silicon oxide (SiO₂) target; a distance between glasssubstrate and target of 60 mm; a pressure of 0.4 Pa; a power of 1.5 kW;an atmosphere of argon and oxygen (¹⁸O) (argon:oxygen (¹⁸O) 25 sccm:25sccm (flow rate)); a substrate temperature of 100° C. The silicon oxidefilm is formed using oxygen (¹⁸O) and thus referred to as the siliconoxide film (¹⁸O).

In Example Sample B, a 500-nm-thick aluminum oxide film was formed overthe silicon oxide film (¹⁸O) by a sputtering method. The depositionconditions of the aluminum oxide film were as follows: an aluminum oxide(Al₂O₃) target; a distance between silicon substrate and target of 60mm; a pressure of 0.4 Pa; a power of 1.5 kW; an atmosphere of argon andoxygen (argon:oxygen=25 sccm:25 sccm (flow rate)); a substratetemperature of 250° C.

In Comparison Example Sample B, a 100-nm-thick silicon oxide film (¹⁶O)was formed using oxygen (¹⁶O) over the silicon oxide film (¹⁸O) by asputtering method. The deposition conditions of the silicon oxide film(¹⁶O) were as follows: a silicon oxide (SiO₂) target; a distance betweensilicon substrate and target of 60 mm; a pressure of 0.4 Pa; a power of1.5 kW; an atmosphere of argon and oxygen (¹⁶O) (argon:oxygen (¹⁶O)=25sccm:25 sccm (flow rate)); a substrate temperature of 100° C. Thesilicon oxide film is formed using oxygen (¹⁶O) and thus referred to asthe silicon oxide film (¹⁶O).

In Comparison Example Sample C, a 100-nm-thick In—Ga—Zn—O film wasformed over the silicon oxide film (¹⁸O) by a sputtering method. Thedeposition conditions of the In—Ga—Zn—O film were as follows: an oxidetarget whose composition ratio is In₂O₃:Ga₂O₃:ZnO=1:1:2 (molar ratio); adistance between silicon substrate and target of 60 mm; a pressure of0.4 Pa; a direct-current (DC) power of 0.5 kW; an atmosphere of argonand oxygen (argon:oxygen=30 sccm:15 sccm (flow rate)); a substratetemperature of 200° C. It is preferable that argon and oxygen used fordeposition of the oxide semiconductor film do not contain water,hydrogen, and the like. For example, it is preferable that the purity ofargon is 9N and the dew point of argon is −121° C. or lower (theconcentration of water: 0.1 ppb or less, the concentration of hydrogen:0.5 ppb or less), and that the purity of oxygen is 8N and the dew pointof oxygen is −112° C. or lower (the concentration of water: 1 ppb orless, the concentration of hydrogen: 1 ppb or less).

Heat treatment was performed on Example Sample B, Comparison ExampleSample B, and Comparison Example Sample C at temperatures in the rangefrom 150° C. to 800° C. The heat treatment was performed in a nitrogenatmosphere under atmospheric pressure for 1 hour.

The concentration of oxygen (¹⁸O) was measured with SIMS on ExampleSample B, Comparison Example Sample B, and Comparison Example Sample Cwithout heat treatment and after being subjected to the heat treatment.

FIG. 9 shows profiles of the oxygen (¹⁸O) concentration in ExampleSample B without heat treatment and after being subjected to the heattreatment at 600° C., 750° C., and 850° C.

FIG. 11 shows profiles of the oxygen (¹⁸O) concentration in ComparisonExample Sample B without heat treatment and after being subjected to theheat treatment at 150° C., 250° C., 350° C., and 550° C.

FIG. 10 shows profiles of the oxygen (¹⁸O) concentration in ComparisonExample Sample C without heat treatment and after being subjected to theheat treatment at 450° C., 550° C., and 650° C. Such a concentrationprofile shown in FIGS. 9 to 11 indicates the diffusion state of oxygen(¹⁸O) in the film and thus is also called a diffusion profile.

It is found from FIG. 9 that the diffusion distance of oxygen (¹⁸O) inthe aluminum oxide film is about several tens of nanometers even in thesample after being subjected to the heat treatment at 850° C. for 1 hourand thus the diffusion is very slow and suppressed. On the other hand,as shown in FIG. 11, in the silicon oxide film (¹⁶O), the concentrationof oxygen (¹⁸O) was increased even by the heat treatment at 250° C. andoxygen (¹⁸O) was observed to be diffused widely. Likewise, in theIn—Ga—Zn—O film in FIG. 10, the concentration of oxygen (¹⁸O) wasincreased by the heat treatment at 450° C. and oxygen (¹⁸O) was observedto be diffused widely. In both of the silicon oxide film (¹⁶O) ofComparison Example Sample B in FIG. 11 and the In—Ga—Zn—O film ofComparison Example Sample C in FIG. 10, as the temperature of the heattreatment is higher, the amount of diffusion and the diffusion region ofoxygen (¹⁸O) from the silicon oxide film (¹⁸O) are larger.

From the above-described results, it was confirmed that the aluminumoxide film can suppress (block) diffusion of oxygen into the film evenin the case where a high temperature treatment at 850° C. is performedthereon, and has a high barrier property against oxygen.

Next, results of evaluation on the barrier property of an aluminum oxidefilm against oxygen according to TDS are shown.

Example Sample A was formed as follows: a 300-nm-thick silicon oxidefilm was formed over a glass substrate by a sputtering method, a100-nm-thick In—Ga—Zn—O film was formed over the silicon oxide film by asputtering method, and a 100-nm-thick aluminum oxide film was formedover the In—Ga—Zn—O film by a sputtering method.

Further, for comparison, Comparison Example Sample A was formed asfollows: a 300-nm-thick silicon oxide film was formed over a glasssubstrate by a sputtering method, and a 100-nm-thick In—Ga—Zn—O film wasformed over the silicon oxide film by a sputtering method.

In each of Example Sample A and Comparison Example Sample A, the siliconoxide film and the In—Ga—Zn—O film were consecutively deposited withoutair exposure, and then, heat treatment was performed thereon underreduced pressure at 400° C. for 30 minutes.

In each of Example Sample A and Comparison Example Sample A, thedeposition conditions of the silicon oxide film were as follows: asilicon oxide (SiO₂) target; a distance between glass substrate andtarget of 60 mm; a pressure of 0.4 Pa; an RF power of 1.5 kW; anatmosphere of argon and oxygen (argon:oxygen=25 sccm:25 sccm (flowrate)); a substrate temperature of 100° C.

In each of Example Sample A and Comparison Example Sample A, thedeposition conditions of the In—Ga—Zn—O film were as follows: an oxidetarget whose composition ratio is In₂O₃:Ga₂O₃:ZnO=1:1:2 (molar ratio); adistance between glass substrate and target of 60 mm; a pressure of 0.4Pa; a direct-current (DC) power of 0.5 kW; an atmosphere of argon andoxygen (argon:oxygen=30 sccm:15 sccm (flow rate)); a substratetemperature of 250° C.

In Example Sample A, the deposition conditions of the aluminum oxidefilm were as follows: an aluminum oxide (Al₂O₃) target; a distancebetween glass substrate and target of 60 mm; a pressure of 0.4 Pa; an RFpower of 2.5 kW; an atmosphere of argon and oxygen (argon:oxygen=25sccm:25 sccm (flow rate)); a substrate temperature of 250° C.

Further, Example Sample A2 and Comparison Example Sample A2 were formedby adding oxygen into the In—Ga—Zn—O films of Example Sample A andComparison Example Sample A, respectively. Note that Example Sample Aand Comparison Example Sample A into which no oxygen was added arereferred to as Example Sample A1 and Comparison Example Sample A1.

In Example Sample A2, an oxygen (¹⁸O) ion was added into the In—Ga—Zn—Ofilm through the aluminum oxide film by an ion implantation method. Theconditions of the oxygen (¹⁸O) ion implantation were as follows: anacceleration voltage of 80 kV and a dosage of 5.0×10¹⁶ ions/cm².

In Comparison Example Sample A2, an oxygen (¹⁸O) ion was directly addedinto the In—Ga—Zn—O film by an ion implantation method. The conditionsof the oxygen (¹⁸O) ion implantation were as follows: an accelerationvoltage of 40 kV and a dosage of 5.0×10¹⁶ ions/cm².

In this example, in order to discriminate oxygen which is added from theoutside into each sample film after formation of the sample film fromoxygen (¹⁶O) that is a component of the sample film, oxygen (¹⁸O), thatis an isotope of oxygen (¹⁶O) that is a component of the sample film,was used to add into the sample film.

Example Sample A1, Example Sample A2, Comparison Example Sample A1, andComparison Example Sample A2 were analyzed by TDS (thermal desorptionspectrometry). The TDS spectrum of M/z of 36 (¹⁸O₂) in ComparisonExample Sample A1 (bold line) and the same in Comparison Example SampleA2 (thin line) are shown in FIG. 12A, and the TDS spectrum of M/z of 36(¹⁸O₂) in Example Sample A1 (bold line) and the same in Example SampleA2 (thin line) are shown in FIG. 12B.

As shown in FIGS. 12A and 12B, a peak was not particularly observed inthe TDS spectra of Comparison Example Sample A1 and Example Sample A1into each of which oxygen (¹⁸O) was not added. In contrast, inComparison Example Sample A2 into which oxygen (¹⁸O) was added, as shownin FIG. 12A, peaks attributed to release of oxygen (¹⁸O) were detectedat temperatures from 100° C. to 200° C. On the other hand, as shown inFIG. 12B, no peak attributed to release of oxygen (¹⁸O) was observed inExample Sample A2 into which oxygen (¹⁸O) was added.

From those results, it is found that by heating in TDS measurement,added oxygen (¹⁸O) is released from the In—Ga—Zn—O film in ComparisonExample Sample A2 where the In—Ga—Zn—O film is bare, whereas addedoxygen (¹⁸O) is prevented (blocked) from being released from theIn—Ga—Zn—O film in Example Sample A2 where the aluminum oxide film isprovided over the In—Ga—Zn—O film. Consequently, it was confirmed thatthe aluminum oxide film has a high barrier property against oxygen andthe high barrier property is kept even when oxygen is added through thealuminum oxide film.

Therefore, in the structure in which an aluminum oxide film is stackedover an oxide semiconductor film, the oxide semiconductor film can bemade an oxygen excess state by adding oxygen into the oxidesemiconductor film through the aluminum oxide film, and the oxygenexcess state can be kept even when heat treatment is performed thereonbecause the aluminum oxide film behaves as a barrier film againstoxygen.

Accordingly, the oxygen-excess oxide semiconductor film enables avariation in the threshold voltage V_(th) of the transistor and a shiftof the threshold voltage (ΔV_(th)) due to an oxygen vacancy to bereduced.

Example 2

In this example, the crystalline state of an oxide semiconductor filmwas observed. Further, diffusion of oxygen in the oxide semiconductorfilm was evaluated by SIMS.

Example Sample D1 was formed as follows: a 300-nm-thick silicon oxidefilm was formed over a glass substrate by a sputtering method, a100-nm-thick In—Ga—Zn—O film was formed over the silicon oxide film by asputtering method, and a 100-nm-thick aluminum oxide film was formedover the In—Ga—Zn—O film by a sputtering method.

In Example Sample D1, the silicon oxide film and the In—Ga—Zn—O filmwere consecutively deposited without air exposure, and then, heattreatment was performed thereon under reduced pressure at 400° C. for 30minutes.

Further, Example Sample D2 was formed by adding oxygen into theIn—Ga—Zn—O film through the aluminum oxide film of Example Sample D1. InExample Sample D2, an oxygen (¹⁸O) ion was added into the In—Ga—Zn—Ofilm through the aluminum oxide film by an ion implantation method. Theconditions of the oxygen (¹⁸O) ion implantation were as follows: anacceleration voltage of 80 kV and a dosage of 1.0×10¹⁶ ions/cm².

Further, Example Sample D3 was formed by performing heat treatment onExample Sample D2 in a nitrogen atmosphere at 650° C. for 1 hour.

For comparison, Comparison Example Sample D1 was formed by performingheat treatment on Example Sample D1 in a nitrogen atmosphere at 650° C.for 1 hour without performing addition of an oxygen ion.

Further, for comparison, Comparison Example Sample D2 was formed asfollows: a 300-nm-thick silicon oxide film was formed over a glasssubstrate by a sputtering method, a 100-nm-thick In—Ga—Zn—O film wasformed over the silicon oxide film by a sputtering method, an oxygen(¹⁸O) ion was added directly into the In—Ga—Zn—O film by an ionimplantation method, and heat treatment was performed thereon in anitrogen atmosphere at 650° C. for 1 hour. The conditions of the oxygen(¹⁸O) ion implantation were as follows: an acceleration voltage of 40 kVand a dosage of 1.0×10¹⁶ ions/cm².

In each of Example Samples D1 to D3 and Comparison Example Samples D1and D2, the deposition conditions of the silicon oxide film were asfollows: a silicon oxide (SiO₂) target; a distance between glasssubstrate and target of 60 mm; a pressure of 0.4 Pa; an RF power of 1.5kW; an atmosphere of argon and oxygen (argon:oxygen=25 sccm:25 sccm(flow rate)); a substrate temperature of 100° C.

In each of Example Samples D1 to D3 and Comparison Example Samples D1and D2, the deposition conditions of the In—Ga—Zn—O film were asfollows: an oxide target whose composition ratio isIn₂O₃:Ga₂O₃:ZnO=1:1:2 (molar ratio); a distance between glass substrateand target of 60 mm; a pressure of 0.4 Pa; a direct-current (DC) powerof 0.5 kW; an atmosphere of argon and oxygen (argon:oxygen=30 sccm:15sccm (flow rate)); a substrate temperature of 250° C.

In Example Samples D1 to D3, the deposition conditions of the aluminumoxide film were as follows: an aluminum oxide (Al₂O₃) target; a distancebetween glass substrate and target of 60 mm; a pressure of 0.4 Pa; an RFpower of 2.5 kW; an atmosphere of argon and oxygen (argon:oxygen=25sccm:25 sccm (flow rate)); a substrate temperature of 250° C.

End planes were cut out from Example Samples D1 to D3 and ComparisonExample Samples D1 and D2 obtained through the above-described process,and cross-sectional observation of the In—Ga—Zn—O film were performedthereon with a high resolution transmission electron microscope(“H9000-NAR” manufactured by Hitachi High-Technologies Corporation). TEMimages of Example Samples D1, D2, and D3 and Comparison Example SamplesD1 and D2 are FIGS. 14A and 14B, FIGS. 15A and 15B, FIGS. 16A and 16B,FIGS. 17A and 17B, and FIGS. 18A and 18B, respectively.

Further, X-ray diffraction (XRD) of the In—Ga—Zn—O film was measured ineach of Example Samples D1 to D3. FIGS. 13A, 13B, and 13C are results ofthe out-of-plane XRD spectrum of Example Samples D1, D2, and D3,respectively. In FIGS. 13A to 13C, the vertical axis indicates the X-raydiffraction intensity (arbitrary unit) and the horizontal axis indicatesthe rotation angle 2θ (degree). The XRD spectra were measured with anX-ray diffractometer, D8 ADVANCE manufactured by Bruker AXS.

FIG. 14A is a TEM image at 8 million-fold magnification in the interfacebetween the In—Ga—Zn—O film and the aluminum oxide film of ExampleSample D1, and FIG. 14B is a TEM image at 8 million-fold magnificationin the interface between the silicon oxide film and the In—Ga—Zn—O filmof Example Sample D1.

FIG. 13A shows the XRD spectrum of the In—Ga—Zn—O film of Example SampleD1.

In Example Sample D1, although a clear peak showing a crystal was hardlyobserved in the XRD spectrum as shown in FIG. 13A, an In—Ga—Zn—O filmincluding a crystal having a c-axis substantially perpendicular to thetop surface (CAAC-OS) was observed as shown in FIGS. 14A and 14B.

FIG. 15A is a TEM image at 8 million-fold magnification in the interfacebetween the In—Ga—Zn—O film and the aluminum oxide film of ExampleSample D2, and FIG. 15B is a TEM image at 8 million-fold magnificationin the interface between the silicon oxide film and the In—Ga—Zn—O filmof Example Sample D2.

FIG. 13B shows the XRD spectrum of the In—Ga—Zn—O film of Example SampleD2.

In Example Sample D2, a peak showing a crystal was not observed in theXRD spectrum as shown in FIG. 13B, and a crystal having a c-axissubstantially perpendicular to the top surface of the In—Ga—Zn—O filmwas hardly observed in the TEM images as shown in FIGS. 15A and 15B,either. Those results make it clear that the In—Ga—Zn—O film wasamorphized by addition of an oxygen ion in Example Sample D2.

FIG. 16A is a TEM image at 8 million-fold magnification in the interfacebetween the In—Ga—Zn—O film and the aluminum oxide film of ExampleSample D3, and FIG. 16B is a TEM image at 8 million-fold magnificationin the interface between the silicon oxide film and the In—Ga—Zn—O filmof Example Sample D3.

FIG. 13C shows the XRD spectrum of the In—Ga—Zn—O film of Example SampleD3.

In Example Sample D3, a peak attributed to a diffraction on the (009)face of an InGaZnO₄ crystal was observed around at 31° (=2θ) in the XRDspectrum as shown in FIG. 13C. Further, as shown in FIGS. 16A and 16B,an In—Ga—Zn—O film including a crystal having a c-axis substantiallyperpendicular to the top surface (CAAC-OS) was observed. In ExampleSample D3, the crystal having a c-axis substantially perpendicular tothe top surface was more obvious than that in Example Sample D1, and astack of layers of the crystalline state of In—Ga—Zn—O was observed bothin the vicinity of the interface with the aluminum oxide film and in thevicinity of the interface with the silicon oxide film, showing thatCAAC-OS was formed in a wide region in the In—Ga—Zn—O film.

In this manner, it was revealed that even from an In—Ga—Zn—O filmamorphized by addition of an oxygen ion like in Example Sample D2, anIn—Ga—Zn—O film having higher crystallinity can be formed byrecrystallization by heat treatment.

FIG. 17A is a TEM image at 8 million-fold magnification in the interfacebetween the In—Ga—Zn—O film and the aluminum oxide film of ComparisonExample Sample D1 where no oxygen ion was added, and FIG. 17B is a TEMimage at 8 million-fold magnification in the interface between thesilicon oxide film and the In—Ga—Zn—O film of Comparison Example SampleD1. In Comparison Example Sample D1, as shown in FIGS. 17A and 17B,although an In—Ga—Zn—O film including a crystal having a c-axissubstantially perpendicular to the top surface (CAAC-OS) was observed,the formation region of CAAC-OS was narrower and a clear crystal wasless observed than in Example Sample D3, showing that the crystallinityis lower.

FIG. 18A is a TEM image at 8 million-fold magnification in the topsurface of the In—Ga—Zn—O film of Comparison Example Sample D2 where noaluminum oxide film was provided, and FIG. 18B is a TEM image at 8million-fold magnification in the interface between the silicon oxidefilm and the In—Ga—Zn—O film of Comparison Example Sample D2. InComparison Example Sample D2, although an In—Ga—Zn—O film including acrystal having a c-axis substantially perpendicular to the top surface(CAAC-OS) was observed in the vicinity of the top surface as shown inFIG. 18A, a crystal was not observed in the interface with the siliconoxide film as shown in FIG. 18B, showing that the formation region ofCAAC-OS was narrow. Hence, it was found that the crystallinity as theIn—Ga—Zn—O film is low.

From the above-described results, it was confirmed that an oxidesemiconductor film obtained by recrystallization by heat treatment anoxide semiconductor film, which is amorphized by addition of an oxygenion, in the state where the oxide semiconductor film is covered with analuminum oxide film is a crystalline oxide semiconductor (CAAC-OS) filmincluding a crystal having a c-axis substantially perpendicular to thetop surface, and the crystallinity is high and good, as shown in ExampleSample D3.

Further, Example Samples D2 and D3 were analyzed by SIMS to evaluateoxygen diffusion therein.

FIG. 24 shows the concentration profiles of oxygen (¹⁸O) in ExampleSample D2 (thin line) and Example Sample D3 (bold line). As shown inFIG. 24, the oxygen concentration in the In—Ga—Zn—O film was not changedbetween Example Sample D2 where heat treatment was not performed andExample Sample D3 where heat treatment was performed, and a release ofoxygen from the In—Ga—Zn—O film by heat treatment was not observed. Inaddition, oxygen was diffused throughout the In—Ga—Zn—O film anddispersed uniformly in the depth direction in Example Sample D3 whereheat treatment was performed. From the above results, it was confirmedthat a release of oxygen from an In—Ga—Zn—O film sandwiched between analuminum oxide film and a silicon oxide film can be suppressed even whenheat treatment is performed thereon, and oxygen can be diffusedthroughout the In—Ga—Zn—O film by heat treatment such that the oxygenconcentration becomes uniform in the depth direction.

It was found also from the results of FIG. 24 that owing to the highbarrier property against oxygen of the aluminum oxide film, oxygen addedinto the oxide semiconductor film can be kept in the oxide semiconductorfilm even when heat treatment for crystallization is performed thereon,so that a stable crystalline oxide semiconductor film can be obtained.

Such a crystalline oxide semiconductor (CAAC-OS) film including acrystal having a c-axis substantially perpendicular to the top surfaceas described above enables a change of electric characteristics of atransistor due to irradiation with visible light or ultraviolet lightand the short-channel effect to be further suppressed. Accordingly, ahighly reliable miniaturized semiconductor device can be provided.

This application is based on Japanese Patent Application serial no.2011-100040 filed with Japan Patent Office on Apr. 27, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a semiconductordevice, comprising the steps of: forming an oxide semiconductor film;forming a gate insulating film over the oxide semiconductor film; addingoxygen to the oxide semiconductor film; forming a gate electrode layerover the gate insulating film; forming an insulating film over the gateelectrode layer; and heating the oxide semiconductor film after formingthe insulating film.
 2. The method for manufacturing a semiconductordevice, according to claim 1, wherein an average surface roughness of atop surface of the insulating film is greater than or equal to 0.05 nmand less than 0.5 nm.
 3. The method for manufacturing a semiconductordevice, according to claim 1, wherein a sidewall insulating layercovering a side surface of the gate electrode layer is formed.
 4. Themethod for manufacturing a semiconductor device, according to claim 1,wherein a heat treatment for releasing hydrogen or moisture is performedon the oxide semiconductor film before the gate insulating film isformed.
 5. The method for manufacturing a semiconductor device,according to claim 1, wherein the oxygen is added to the oxidesemiconductor film by an ion implantation method.
 6. The method formanufacturing a semiconductor device, according to claim 1, wherein theoxide semiconductor film is crystallized in the heating step.
 7. Themethod for manufacturing a semiconductor device, according to claim 1,wherein the insulating film is an aluminum oxide film.
 8. A method formanufacturing a semiconductor device, comprising the steps of: forming afirst oxide semiconductor film; forming a gate insulating film over thefirst oxide semiconductor film; adding oxygen to the first oxidesemiconductor film to form a second oxide semiconductor film; forming agate electrode layer over the gate insulating film; forming aninsulating film over the gate electrode layer; and heating the secondoxide semiconductor film after forming the insulating film to form athird oxide semiconductor film, wherein each of a crystallinity of thefirst oxide semiconductor film and a crystallinity of the third oxidesemiconductor film is higher than a crystallinity of the second oxidesemiconductor film.
 9. The method for manufacturing a semiconductordevice, according to claim 8, wherein an average surface roughness of atop surface of the insulating film is greater than or equal to 0.05 nmand less than 0.5 nm.
 10. The method for manufacturing a semiconductordevice, according to claim 8, wherein a sidewall insulating layercovering a side surface of the gate electrode layer is formed.
 11. Themethod for manufacturing a semiconductor device, according to claim 8,wherein a heat treatment for releasing hydrogen or moisture is performedon the first oxide semiconductor film before the gate insulating film isformed.
 12. The method for manufacturing a semiconductor device,according to claim 8, wherein the oxygen is added to the first oxidesemiconductor film by an ion implantation method.
 13. The method formanufacturing a semiconductor device, according to claim 8, wherein thesecond oxide semiconductor film is crystallized in the heating step. 14.The method for manufacturing a semiconductor device, according to claim8, wherein the insulating film is an aluminum oxide film.
 15. A methodfor manufacturing a semiconductor device, comprising the steps of:forming an oxide semiconductor film containing In, Ga, and Zn over asubstrate; performing a first heat treatment after forming the oxidesemiconductor film; forming a gate insulating film over the oxidesemiconductor film after performing the first heat treatment; addingoxygen to the oxide semiconductor film after forming the gate insulatingfilm; forming a gate electrode layer over the gate insulating film afteradding the oxygen to the oxide semiconductor film; forming an insulatingfilm containing aluminum oxide over the gate insulating film and thegate electrode layer after forming the gate electrode layer; andperforming a second heat treatment after forming the insulating film.